The seller did one of two very common things: didn't count the screw under the grip or didn't count the "4th" screw in front of the trigger guard. This probably means it would not have had an appropriate reserve no matter how many screws were counted.
Ironically, my buddy and I ran across the mate to this gun in 4", w/ cokes and serial # 168XXX
at this weekends local show. He bought it for $1200. Although it was over serial # 167XXX, it still only had 6 grooves on rear of backstrap, not 10.