I have a (1951) 336SC and a (1949) 336A and both have a milled channel in the barrel and magazine tube for bolting on a front swivel.
Why would Marlin do that if the general consenus is they didn't come with swivels. The swivels on both of my guns are identical. The pic
that is circled is of the milled channel (hard to see). Am I missing something.

Pretty sure that wasn't done by Marlin I have never seen a Marlin like that,I can't find anything in my Marlin book that that was ever done
Must be some after market set up back in the day looks good though.

Is the milled out hole blued or is bare metal showing that would be a tell tail sigh it was done after market.

Did you get both guns from the same person?

No, I have seen some posts claiming they purchased theirs with factory swivels. Most people say different.

What gets me is both sets of swivels are identical.

Some older Marlin 336's came from the factory with sling swivels mostly the deluxe 336's with the end cap swivel like the SC and A models.

I am looking at a .35 Rem 336 with that same swivel on it. I may pick it up tomorrow