Duet concertinas share the Wheatstone ledger coverage with English models; Lachenal duets used their own formula. Identifying the system comes first.
On the right-hand end plate or an internal reed-pan label. Markings and exact positions vary by maker and era — the serial number guide covers the common locations for every instrument.
6 makers with 378 dating ranges are listed on this page. Coverage grows as more maker records and historical tables are verified.
Recheck the characters exactly as marked — letters and leading zeros are part of the number. On several makes the stamped numbers are internal assembly or importer codes that cannot be dated; some production periods are simply not covered yet.