Engine 49guy,
It's a strange gun. I bought it because it was just cool. In researching it, I got even more confused. It's a 629-1 with a S/N BAN8XXX which would make it around early 1988 production. I called S&W with the serial number and was transferred to the guru himself, Roy Jinks, and spoke to him at length (nice guy).
He said it was a product code 103619 and was one of 40 (not 20) consigned by RSR Group in Aug. 1998. They were designated as "Special Mountain Gun" with square butt, tapered barrel, bead blasted finish, pinned front sight and this one has smooth Goncalo Alves Magnas.
No idea why a 629-1 frame of 1988 vintage was resurrected as a 1998 Special Mountain Gun - but who knows for sure. (Roy said S&W is very "Frugal" and assumed they had enough extra 629-1 frames in stock for a small production run).
(I've sent for a letter of authenticity from Roy)