Many Stevens rifles and Stevens barrels have passed through my elderly hands. I concur that this is a Stevens barrel (never a doubt), but I doubt that it is a take-off. The Steven-installed barrels I have seen have multi-digit numbers stamped (aligned with the bore) just forward of the forearm. None present here. I think that Stevens did sell new unnumbered barrels. I'd advise the OP to slug the barrel, measure the twist, and then make a chamber cast.
I have a Stevens 44-actioned Model 47, marked 25-20 just like this. The chamber is 25-20 SS, but the groove diameter is .251" and the twist is 17. The subject rifle may or may not be chambered for a "25-20" cartridge.
