The First Model American has a hammer without the notch for the latch and has a hammer activated cylinder stop.
ALL 1st Contract Imperial Russian revolvers have the notch and a hammer activated cylinder stop. The hammer with the notch was introduced into the commercial production some where between serials 6466 and 6744. This hammer also featured the hammer activated stop.
The trigger activated cylinder stop was introduced with the 2nd Russian Contract and the Second Model Russian revolver.
The trigger activated stop was introduced into commercial production revolvers at about serial range 18634-19175. This new hammer was shaped like the earlier commercials with constant thickness, but without the lug for the hammer activated stop.
In any case the hammer found in the late 1st model (3rd model American?) production would likely interchange into a Second or Third Model Russian.
I cannot see Smith & Wesson producing Third Model revolvers with the old hammer style unless it was a special order. My suspicion is that the early hammer in a Third Model would be a replacement part and not original.
hammer detail 1.jpghammer detail 2.jpg
Hammer details from a 1st Contract Russian (First Model Russian) and a 2nd Contract Russian (Second Model Russian)
Joe