Pipe-Organ Opus Numbers and Date Evidence

An opus number identifies an instrument or job in a builder’s own series. The definitions an owner is most likely to meet put it that way: a congregation’s history of its Wicks instrument, Opus 3296, describes the number as a running count and calls it analogous to a serial number, and the Garden State Theatre Organ Society glossary defines opus as “the serial number of the instrument.” Both are owner-side or association explanations rather than a builder’s statement of its own numbering policy.

A serial number, strictly, belongs to a product series, and the same glossary keeps the two apart: it defines a blower number as the serial number of the blower that powers the instrument, and style as Wurlitzer’s term for the model. An opus number identifies the instrument in the builder’s series. It does not identify a component and it does not name a model.

Around the number, builder lists, archive finding aids and trade-press accounts use a fairly settled vocabulary: opus number and opus list; contract file, contract-signing year, shop records, financial volumes and design specifications; dedication program and stoplist. The same material names the nameplate, signature plate, company plate and builder plate, and the keyslip, nameboard, stop jamb and commemorative plaque that carry them. Opus lists also use notations such as NB for not built and NR for no record, and suffix forms such as -A, -B and -C. Each of those terms points to a different evidence question, taken up on the pages linked below.

A match returns the fields the list records for that entry and nothing more. Austin Organs is the clearest current example. As of August 31, 2026, Austin instruments carry opus numbers, and the searchable opus list on the manufacturer’s website matches a number to a location, a venue, a number of manuals and a number of stops. Where the number on your instrument is legible and falls within the list’s coverage, the match can establish those listed facts.

If a list also gives a year, read the list’s own note on what that year records. Where the list does not say, treat it as a listed year, not a build date, until you have worked out which event it stands for.

Casavant Frères announced its first Inspiration Organ in the United States, installed in December 2020 in the restored Crawford Chapel at Rye Presbyterian Church in Rye, New York, and identified the instrument in that announcement by the designation II-24/9 rather than by an opus number. The announcement does not say whether an opus number was also assigned, and it does not decode the designation, so its meaning should not be inferred from the format alone. The instrument has a documented identity either way; the number is simply not the record that carries it.

Sometimes the silence is in the record rather than in the announcement. For the Reuter Organ Company, the published and archival record available as of September 1, 2026 does not state where the firm placed a builder’s plate or an opus number on a console, windchest, reservoir, pipework or any other component, or what such a plate looked like in any era. That silence runs across the firm’s own website, trade press, congregation and dedication accounts, other firms’ rebuild reports, institutional instrument documentation and the Kenneth Spencer Research Library finding aid alike. It does not establish that the firm had no convention, only that the published record, as of that date, does not describe one. Where plates and numbers physically sit on an instrument is a question for inspection, not for the documents.

A number records sequence, a date records an event, and only a document ties the two together. Two documented number-and-year pairings show the limit.

The manufacturer’s own history states that the Great Depression forced the original Austin Organ Company to cease production by 1935, and a tertiary encyclopedia article adds that closure was announced by 1935 after the company completed Opus 1885. That pairing tells you where the Austin series stood when production stopped. It says nothing about when any particular number below 1885 was contracted, completed or installed, and it does not mean numbers were completed in strict sequence.

Casavant Frères states in its own published timeline that its first United States installation was opus 63 in Holyoke, Massachusetts, in 1895, and the Dictionary of Canadian Biography states that the firm began exporting instruments to the United States in 1895. The two sources agree on the year, but neither reproduces a contract, a shipping record or an installation certificate. The pairing rests on a company timeline and a scholarly biography, not on the record of a dated event.

This is how a year gets attached to a number in practice. An Organ Historical Society convention handbook establishes the provenance of the instruments it describes from documentary sources such as a contract date and factory ledgers rather than from physical markings on the instruments. That reflects the sources its authors used; it is not evidence that those instruments carry no markings. Examples like these establish the method and its limits. They prove nothing about your instrument without that instrument’s own records and physical evidence.

Different documents record different events, and later summaries copy from different documents. Firm-level dates show this plainly.

For M.P. Möller, several sources give a founding of 1875 in Greencastle, Pennsylvania; one detailed reference history instead describes a Greencastle partnership named “Möller, Brenisholtz and Co.” running from about 1877 to 1881; and the Organ Historical Society’s own finding aid for the company’s business records says the company was “established in 1881.” These accounts may refer to different stages of the business or may contain errors. Keep each date attached to the event and source that reports it.

For the Reuter Organ Company, the National Register of Historic Places registration form for the company’s buildings states that the firm was incorporated in Kansas on 15 September 1919, while Wikipedia gives 5 September 1919. The registration form is a preservation document rather than the corporate filing itself, so September 1919 is as precise as the incorporation date can be stated without the filing.

Both are firm-level dates, not instrument dates. They show that dated records disagree and why; they do not show how often instrument records disagree. For an instrument, the same question takes the form: is this year a contract year, a completion date, an installation date, a dedication date, or the date of a later console or rebuild?

You have a year from a record. Contract Date Versus Completion Date explains what a contract date, a completion date, an installation date and a dedication date each establish, how far apart they can sit for one instrument, and which to quote when a record gives only a year. Read it before comparing two sources; a comparison means something only once you know which event each source reports.

The date is on or inside the console. Why the Date on a Console May Not Be the Date of the Organ covers whether a date found there belongs to the console or to the instrument it controls, and what has to be true before a console date can be quoted as an instrument date.

The instrument carries more than one name or number. Rebuild Plates, Secondary Opus Numbers, and Layered Instrument Identities explains what a second nameplate, a rebuild plate, a suffixed number such as -A or -B, or a number from another firm’s series each record, and how to describe the instrument without collapsing its layers into one name.

The number is not an opus number. Shop Orders, Job Numbers, Contract Numbers, and Other Non-Opus Identifiers covers job numbers, contract numbers, shop-order numbers, blower numbers and short stamped figures whose class you cannot name: what record each retrieves and why a number from one class cannot be read as a number from another.

The same number appears twice. Why Two Builders Can Share the Same Opus Number explains why independent firms’ series overlap, how one firm’s name rendered differently across databases produces false matches, and what to check before a bare number is taken as evidence of who built the instrument.

You are working from a published list. How to Read an Organbuilder Opus List covers what a list entry establishes, what a list says about its own scope and currency, and how to read gaps, paired numbers and notations such as NB and NR.

When the records run out, the remaining questions are answered on the instrument itself, through inspection of its plates, marks and components, or through a request to the repository that holds the builder’s records.