The rear sight should have the serial number. Perhaps one of the RM gurus can confirm or object to that. If Serial number (that's my reasoning) this number is likely off a triple lock in that serial number range. The target sight assembly from a triple lock appear identical to those on a RM. I've seen this same scenario before. I don't want to "presume" but all those large frame targets of the era all seem to have the same style size rear sight. Triple lock, 2nd model HE 44, 3rd model HE 44, 38-44 HD (which was the predecessor of the RM)
Interestingly, this whole experience taught me a lot about pre-war rear sights.
In my digging I've now seen 3 pre-war target sight variations. The Triple Locks, and early rear sights seem to have 1 screw to the rear, the later ones have 2 screws, the Registered Magnum rear sights were like the later ones with the addition of the horizontal grooves to the rear.
I don't own an early triple lock, but I did find one triple lock or possibly early 2nd model rear sight for sale.
This is the RM rear sight grooving:
Here's the rear sight on my 38/44 OD without the grooves:
This sight will fit on the registered magnum, and would work, but it is wrong.
I can't find a good picture of the rear sight on a triple lock, I expect someone can supply one.
As an interesting side note, the K frame rear sights appear to be exactly the same as the N frame, with the difference that the tang is of a different length.
All of the pre-war sights I have come up with are numbered.