I have a 992 movement, identified as such by the serial number, dating from 1906-1907, that does not have an identifying model number on it. Other later 992’s I have are identified on the train bridge with the model number.
When did Hamilton begin identifying model numbers on the movement?

I think they started marking them with grade numbers somewhere around 1913-14, someone with more knowledge then me on Hamilton's will chime-in before long

We have a 1909 that does have the 992 grade number on the train bridge.