I would bet it is an aftermarket conversion. Too bad, I could use an ACP frame.
It is not as simple as swapping the barrel and cylinder. The frame lug would need to be changed also. Not a tough job but noticeable if not done.
I converted a Model 28-2 to a 4”, 45 ACP. I had the 357 barrel bored and rifled for the ACP bullet, fit a new cylinder and frame lug while the barrel was gone. When the barrel got back it needed to be turned in a thread or two to make the barrel cylinder gap acceptable. Two turns was too much so the barrel shank had to be shortened. I did the work myself but unlike Steelslaver, I used hand tools. Mostly a portable hand held milling machine, ie, a hand file. The total into the conversion was well under $200. I want to say under $150 but do not recall. I know it was not much.
Kevin