Seems like there should be a model number in the yoke area. I recently bought 695XXX which is a nickel round butt 32 Long with four inch barrel shipped in early 1961. Model numbers were stamped starting in 1957 so I would think 677721 would have been shipped after 1957. Mine is stamped in the yoke area as a Mod 31 but an S&W letter confirms that it is a mis-stamp and shipped as a Model 30. Anyway, the one you're considering is likely a Model 30 if round butt or a Model 31 (Regulation Police) if a square butt. Great little guns in my opinion. $500 sounds a little high without the original stocks but not much. Good luck.

Jeff
SWCA #1457