The early Model 121s did have the Ilion Works stamp on the barrel. Most of my rifles are not available right now, but the Ilion Works stamp appears to be pre-war. I found several from 1936 and one from 1938 that were so stamped. 1954 was the end of production.
I don't know about the other models.
John Gyde
121 Fieldmaster - Unusual barrel rollmark
M/121
I believe that the "3" will be stamped with a new barrel code indicating when the repair was done??? Mine appear to indicate this. The barrel code was stamped during final assembly so the lack of a code on your 550 barrel means it was never installed at the factory. I can't shed any light on the barrel code you indicated.
John
John