From my search of dated SN here I'd say it was made (accepted) in January 1864, but wait until Mike Strietbeck comes along, he will know for sure.
Have you removed the grips and trigger guard?
Serial number is found on the grip frame, on the ínside of the trigger guard and on the underside of the barrel. If the serials don't match up then something has indeed been replaced.
Intresting that you only have "W" sub-inspector marks.
My own NMA seems to have most of the alfabet on it.
Cheers!
/Anders
Oh! and...
