I agree with Gil, we need more pictures. If it is an overstamp, that's fine, but the only fixed sight .357M N frame is the M520 and that's not what was the original stamp. There was a recent thread about overstamped M58 frames that became M520's made out of the original production run.
M58-1 is a second-production revolver. Original production stopped in 1977, and the dash-1 started in 2009 as part of the Classic Series. It has a CNC frame and the Hillary Hole. If you're looking for a M58 I would keep searching for a no-dash, which all the original production guns are.