I’d like to see the engraving on the Franklin barrel.
Looks like the barrel was cut off. Then that "extension " crimped on over it. I suspect they may have cut off the receiver end of the barrel also. Welded barrel shut. then refit that "threaded nut" in and attached only a short piece of the original barrel.
Since the pistol was in England, you can bet it was stamped a lot more before it could be sold. Proof marks included...Wow, that's actually a great theory towards threaded portion! I hadn't thought about that.. The only issue though is the markings on the barrel, most stock Rugers pre-'76 didn't have any barrel markings that I've seen, so possibly another barrel but with a threaded adapter as you mentioned, maybe? I'm still not sure why they even changed the barrel at all and turned down the upper (roughly an 1/8" to .13" from what I can measure from the photo)..
The only thing I can think of is that they wanted to remove the stock barrel from that pistol they had already in their armory a ways before, and at some point cut the upper short to loosen the tension of the super torqued barrel more easily..since Ruger was kinda known for torquing their berrels fairly tight, haha. With Bapty's extensive inventory, turning a receiver down to remove a barrel quickly wouldn't be much of a stretch I suppose, they were known for modifying and machining pretty much anything and everything that they wanted to on a whim. An interestingly sad freedom I suppose..