Its is what S&W calls a 657-4. The 6 = stainless steel, the 57 = .41 magnum & the dash 4 @ the end = 4 modifications done from first design.

But this one has a 4" barrel with a fast taper to save weight, & has the N-frame round butt grips.

Thats what makes it a Mountain Gun, it's just lighter than your stock 4" N-frame magnum

c/22
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Apocalypse Steelheader.
Chucking gear as the end draws near.