1961 (M1) - 23 Jewels
I have a 23 Jewel 10BZAC movement with an "M4" date code.
Looking for pre 1959 example. None in our database. Rantff remarks "1959 to 1963" at http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&&2uswk&Bulova_10BZAC. NOVA also lists start date as 1950, but her two 23 Jewel examples are from 1959 (Beau Brumme)l and 1963 (Regatta). I think the 1950 estimate is too early.
Edited to start date of 1959 w/ this caveat. End date changed from blank to 1963.
Added to data base 11/24/2011 year is estimated as of this date. Could be mid 50's.