Hello---
I am new to this site, but I am a fan of the 219/220, have a few, and hope to be able to contribute a bit to the discussions.
As to the interchangeability of fore-ends, I don't believe that any shotgun fore-end (except for a .410) will properly fit on a rifle barrel. It may snap on, but there is bound to be a gap between the wood and the barrel, increasing towards the muzzle.
Comparing the fore-ends that I have, I believe that the channel diameters were different for the 12, 16, and 20 ga. I do, though, believe that the channel diameters of the .410 and all rifle calibers were the same.
I also see several different designs in the fore-end hardware. That of what I think is the earliest has a clipped-off washer under the screw and a curving, doubled-over spring-piece. The next has the same spring piece but no washer under the screw. Instead a cone shape. The third type has the cone shape but the spring piece is not doubled over and is not curved but wavy. These fore-ends are all from Utica guns.
If I were looking to buy a fore-end to add to a particular gun, I think I would try to find one that had the same kind of hardware as that that the gun already has.
Incidentally, I also note that there are two different types of hooks (not sure what better word to use) on the barrels. The earlier type is heavier. The later type is slimmer, lighter looking. But I suspect that this is not an issue. I believe that either type will serve to clasp the barrel to an otherwise proper fore-end.