Prior to WW2, the default finish - meaning no marking on the frame as to finish - was nickel, and a B would be stamped to indicate a blue finish. During WW2, no nickel guns were being made, so to speed up production, the default finish finish became blue. Following the end of WW2, and the resumption of nickel finish, the default finish remained blue, and a N was stamped, typically under the grips, and also on the barrel, to indicate a nickel finish. As Bill noted, the trigger ( and probably the hammer ) are white in color, indicating a non-factory refinish.

Mike Priwer