sm - - To expand on the answer by Tamara - -
The 21-4 Thunder Ranch revolver is an "update" of the M21 discontinued several years back. This was catalogued as the 1950, which was, in turn an updated 1926, Third Model Hand Ejector. The plain ol' 1950 had fixed sights and tapered barrel without rib. The 1950 Target came to be designated the Model 24, and had adjustable sights with ribbed barrel, the rib being somewhat narrower than that on the model 29 - - Looked a lot more like the rib on the models 27 and 28 .357s.
Tams -- my apologies. I shouldn't have even TOUCHED on that subject in his forum, even with the mention that it belongs on L&P. Heck, I'll go back and self-edit . . .
Best,
Johnny