In over 50 years working on S&W revolvers I have never seen that. May have been something like the "floating" hand, or gas ring on the yoke that didn't last long. Maybe the transition between the one-piece and two-piece bolts? The other, more likely, explanation is it was a "field expedient" repair for either a bolt that "Bubba" shortened because he didn't understand how the gun unlocked, or to repair a bolt where the pin had broken off some way. Again, probably Bubba was probably involved. Later bolts are two-piece, where the pin is pressed into the bolt body, instead of being machined from a single piece as earlier ones were.