Others would know more precisely, but from the SN definitely pre-1920, maybe pre-WWI. Also, grips are of the 1910-20 period. Very nice. I don't know who would be able to tell you the plating thickness, but there are ways of measuring it (platers know how). I wouldn't use any type of aggressively abrasive cleaner, but you might try automotive paint rubbing compound (the white paste) in a small area as a test. At least the hammer and trigger do not appear plated, and that's a good sign.