Oops, sorry, I misunderstood.
Still, I don't know where to match the serial number to a specific or approximated date. All I know is that there were some 35.000 Bessas II made, and that the accessory flash shoe was included from 1954 onwards (apparently, the Bessa II was discontinued by 1956).
If you ever find out let me know, it may be a nice resource 🙂