From 1969 to the end of 1973 BSA used a new system for coding frames and engines.
BSA manufacturing year ran from August to July. The production of, say, a 1969 model began in August 1968 and finished in July 1969.
Two letters and a serial number followed with model type. All model types started with No 00100
Ex. BG 1234 B50SS is made in February 1972 (1972.02)and model type is B50 SS.
Engines were built first, given a number and stored.
All OIF unit singles were in the same sequence ie hypothetically engine number 4999 could have been a B25SS, 5000 a B50 and 5001 a T25SS (Triumph).
The production of complete OIF machines was delayed (apparently due to frame manufacturing difficulties) and although engines are dated from the start of the model year (say August/September 1970) bikes didn't start to roll until early in 1971.
On some later models the date of Manufacture of the crankcase is stamped near the rear mounting lug.