From the Concertina FAQ (www.concertina.info, linked above): Appendix 1: Wheatstone and Lachenal Dates of Manufacture.
The Wheatstone ledgers are at this site (for 1910-1923; Serial No. 25000 to Serial No. 29749, 9 May 1910 to 12 November 1923).
~ Becky in Tucson