Actually, looking at the location of the pronounced shoulders in the chambers in the cylinder photo, I'm fairly certain it's the original .38 S&W even without the cartridge test.
I concur. The only reason that I'm not 100% on this is that it's hard to tell whether there may be some "foreshortening" in the viewing angle. I think you are right, which makes it even more interesting; if one were going to the trouble of putting in a new cylinder, why not put in a 38 special cylinder?
Of course, we need to OP to try dropping in a 38 spl cartridge, and if he can find one, a 38 S&W cartridge, and see which one fits.
Best Regards, Les