Welcome to the forum.
The 'M' prefix indicates that your SPORTSMAN 1st Model 8th Variation was made in 1952 - the last year of production with the bird's head shaped Rice type frame and one piece grip.
The value on the SPORTSMAN series revolvers has more than doubled over the past few years - demand is way up there. If your gun is New In the original Box (NIB) it would RETAIL for $400+, up to maybe $450 on a very good day. In lesser condition it will sell for $300+ in Good (Average for used) condition (about 85% finish left). It drops off down to around $225-250 if in poorer condition than that. You don't state condition of your piece, so the RETAIL value will fall somewhere in between.
From experience shooting 1st Model vs 2nd Model (1953-72) to 3rd Model (1973-86) and then the reconstituted H&R 1871 versions, I've found the 1st Models to be the best built, best fitted, best finished and most accurate - I hope you find that your piece is the same.
Here's a pic. showing the difference in grip style from 1st Model to 2nd Model.