I agree with Pioneer. Probably made in the last few months of '56.
1956: K266165-K288988
I love my S&W 38 Specials, also an N-frame 5 inch fixed sight, Heavy Duty made in 1940, and used by the BCPP & at the other end,
a K-frame in stainless steel, Model 64-3 with a retrofit six inch target barrel circa 1985. (I am not 12. anything)