Dean:
M stands for mix (called universal) purpose in the catalog, H stands for hour and T stands for tachy, calculators were slow sellers at the end of the lifecycle hence the additional model, no date frame new dial wihout Calculator, both bezel types were available for the black, similar to Autavias, Carreras, Monacos, Verona, Monzas all was a little mixed those days, I also own a nos Calculator with M case and have seen a few others. 315xxx and 316xxx were the later numbers, the one in my book page 80 sold on June 1982 through Chronoswiss...bezels were interchangeable, cases often no serial number, was put in by service exchange, some case batches came without bezels.
Best Arno

: Can someone explain the difference between 110.633 and the 110.633
: M models and is it possible to date a calculator based on serial
: number (e.g. 298214) ?
: Thanx