First, there two relevant NM BH cylinder frames that you will run into. The medium frame is found on the flattops (.357, .44 Special, and .45 Colt) . The exception 'flattop' is the .44 Magnum 50th Anniversary which is on the large frame. All other NM BHs and SBHs are on the large frame since the New Model was introduced in 1973. Easy to tell as the New Model large frame will have 'ears' around the rear sight (not a flattop). Doesn't matter the grip frame type... just the cylinder frame. Hope the 'summary' helps you decide what you have!
Also as a quick how-it-is. The Old Model (also know as the 3 screw model) BHs started production in 1955, ended in 1973. Note that the OM .357 BH was on the medium frame. All others were on the large frame. The New Model (two pins though side the frame, and a transfer bar) from 1973 to present. The .357 went to large frame in 1973 to join the others. The NM flattops came much later starting with the .357 50th Anniversary flattop in 2005. The .44 Magnum 50th Anniversary flattop in 2006. Then the .44 Specials, and finally the .45 Colt/.45 ACP (wahoo!) medium frame flattops.
Then there is the side case of the .357 Maximum on a stretched Large BH frame.... There is always the 'odd ball'
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Just a 'summary' history. More to the story of course
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You didn't say what cartridge your gun shoots ....