It's Ithaca themselves that is feeding that "line of BS."
OP, that's correct. Not all Ithaca M37 barrels are interchangeable, and not all barrels that are interchangeable end up with the correct head spacing.
I have an older 37 (1957). When I was looking for a slug barrel Ithaca told me that all 37s made before a
certain year would accept any barrel made before that year but that any 37 or barrel made after that year needed to be fitted. Unfortunately, I do not remember the specific year.
