There are 4 SMG upgrade trees tied to the Assaulter II Axis squad…the MP35/I, the Baretta M1918, the ZK-383 and the Baretta M38 (in that order). Same for the Allies (Sten Mk II, Winchester Model 1912, Lanchester, and M1A1 Thompson).
57 levels…which I think is the longest grind in the game (so far).
I think at least 1 of those SMGs for both Allies and Axis could be peeled off to either Assaulter I or Assaulter III.