Sir.
I have some S prefix N frames and K frames. That makes then older than the 2 you mention.
The screw in most of mine that holds the yoke/crane in does not have anything on the end of it. It has been adjusted to a correct length at the factory and should always go in that same hole.
The more modern guns may be different, I think my two newest guns, 25-7, do have a different type screw in that hole.
Bill@Yuma
RdrBill is right. Your S model should have a round head sideplate screw that was fitted at the factory to the yoke. It is the same screw as the other round head sideplate screw but that one is NOT fitted to your yoke, so never interchange them.