I'd like to know whether that was one company that did all of these, too.

The Cogswell & Harrison conversions are a completely different animal, only shortened so that the underbarrel lug remained, and always proofed in London for the .38 Special after the conversion.

The ones Charles is referring to were almost certainly done here in the US. Where British proofmarks are visible, they indicate Birmingham proofing for the original .38 caliber. Sometimes the grips are marked Franzite, but more often not.

It's interesting that Charles says all those at his pawnshop are still in .38 S&W. That I have never encountered before on a gun that had gotten the barrel-cut, nickel-plated, fake-stag treatment. Cylinder reaming usually came first.