One theory is that the army stopped giving model numbers as the year of adoption because you didn't want anyone thinking they had something old, although it doesn't seem to bother anyone who likes .45 autos. That's what they used to be called, you know. However, the year of adoption continued to be used in some cases, though not for firearms. There was the 1956 model webgear and the 1952 field jacket and so on. In some cases model numbers seem to be applied retroactively.
A similiar thought process seemed to be at work when Winchester quit calling it a Model 1894 and using just Model 94 instead. But I've suspected the Model 88 had more to do with the Model 99 Savage (formerly the Model 1899!). Someone just had to come up with a Model 77, too, didn't they?
I think US Army rifles were officially, "U.S. Rifle, M14," etc.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.