There should also be a SN on the face of the cylinder that matches under the barrel. You need to have those numbers, if they are identical, stamped or etched on the grip frame. In its current condition, the gun is not legal per the GCA of 1968.
Almost any S&W N frame is worth $500 if it functions. I believe what Geezer meant is that the value of that gun is worth more as an heirloom than it could bring in a sale. That's particularly true now because you can't sell it...legally.
Thank you Guy, that's exactly what I meant.................
Angusnstep, as Guy has mentioned, by law the serial number must be present somewhere on the main frame of the gun, If it's on the underside of the barrel and also on the cylinder face, it must still be on the main frame somewhere. I strongly suggest that you carefully remove the grips (take care not to bugger the grip screw) and look at the sides of the grip frame (where you can't see while the grips are on) to see if someone inscribed the serial number on it. The grip frame is part of the main frame so it satisfies the law if the serial number is inscribed there. Normally the "official serial number" would be on the bottom of the butt, but it would appear that it was removed when the revolver was refinished,
If the serial number on the bottom of the barrel matches the serial number on he cylinder face and not on the main frame somewhere, then you may want to inscribe that same number on the side of the grip frame (under the grip so as not to deface the frame). FYI, if the barrel serial number does not match the cylinder face number, there's a good chance it's a "parts gun" (made of parts from several guns)....................If you suspect that it's a parts gun, then two considerations surface.......1) "was it properly and safely put together?" 2) "Could the parts have come from a de-milled gun and if so, hopefully the serial number doesn't represent a gun that was confiscated and the number remains on file somewhere.
I certainly don't want to come across as being overly cautious.....but these are important legal considerations and the negative consequences are not worth you getting wrapped up with........and it also affects the value.
Best Regards,
Geezer