This is pretty simple to figure out because the gun is stamped WS1.
I have never seen a genuine special order 24" M-12 barrel. But if this is "real" the bore mic will demonstrate it clearly. WS1 was a proprietary design with constriction followed by flare at the muzzle. In a 20 WS1 constriction usually amounts to about .003 or so, maybe .004. I have not measured one (20 gauge) at .005.
If the barrel was orignally 26" (most likely) there will be no flare following constriction. Indeed, there will not likely be much constriction at all.
Sam
