Unscrew the front sideplate screw, open the cylinder and pull the cylinder and crane off to the front. See if there is a spring and plunger in the hole in the crane axle (the part that fits into the frame). I'll bet there isn't; the parts were probably lost when someone took the gun apart.
Anyway, that spring and plunger operate the cylinder stop. You can probably get the parts from
www.gunpartscorp.com.
Jim