First off the 651 has adjustable sights and the 650 does not and I clarify this because there seems to be a significant difference in how buyers value these with the 651 bringing more dollars.
The SCSW says "Also offered with an auxiliary .22 Long Rifle cylinder in early production." It does not clarify if it's the no dash or the dash-1 but in context it appears to be referring to the no dash 651.
Here's what SCSW shows:
103900 2" S RB with Boot Grip RSR Special, 1999, 1241 reported manufactured.
103900 Same code as above; also reported for 1998 Lew Horton Production.
A quick review of Gun Broker for successful auctions that actually resulted in a sale for S&W Model 651 show the following ( I did not report all sales):
651-1, 2", looks original, with box, does NOT have extra cylinder, product code 103900, rubber grips, blue plastic S&W box, LNIB, $1525.
651-1, 2", looks original, with box, does NOT have extra cylinder, product code 103900, rubber grips, blue plastic S&W box, LNIB, $1625.
651-1, 2", looks original, withOUT box, does NOT have extra cylinder, product code 103900, rubber grips, blue plastic S&W box, excellent, $1119.09
651-1, 4", looks original, with box, does NOT have extra cylinder, rubber stocks/grips, excellent, $860
651-1, 2", looks original, box in picture may not be original (1 piece cardboard), rubber stocks/grips, excellent, does NOT have extra cylinder, 906.98
651 no dash, 4", original, with box, both cylinders, excellent to LNIB, wood stocks/grips, One piece cardboard S&W box $2195.
651 no dash, 4", original, NO BOX, excellent condition, wood stocks/grips, $825.
651 no dash, 4", original, NO BOX, excellent condition, wood stocks/grips, 960.01.
My opinion, without pictures or more details, is $900 out the door with the extra cylinder is fair, if in excellent condition, and if it has the original box it's more than a fair price. I bet the rear sight cleans up well (?) - pictures would help with a better guess.