Driftwood Johnson, on the two Smith and Wesson revolvers, on the cylinders, there appear to be 2 sets of circumferentially machined slots both for the locking bolt, and then another set on the cylinder in front of the locking bolt slots. What is the purpose of the secondary slots? Is it a double lock of sorts to keep the cylinder firmly in place?
I don't want to hijack this thread, but I will attempt to answer your question. The S&W 44 Double Action had an unusual cylinder locking system. Yes, there were two rows of locking slots.
There was a relatively conventional spring loaded bolt that engaged the rear set of slots. But in addition to that, there was a raised nub on the forward part of the trigger that engaged the front set of slots.
The mechanism worked a bit differently than most revolvers. With most revolvers, the bolt is withdrawn into the frame as the hammer is cocked, allowing the cylinder to rotate. At some point the bolt pops back up under spring pressure, rubbing against the lead in to the locking slots, and popping into the appropriate slot to lock the cylinder in battery just as the hammer reaches its farthest backwards movement, either single action or double action. The bolt then stays up under spring pressure as the hammer falls, and the mechanism resets to allow the bolt to be withdrawn again for the next shot.
The 44 Double Action was different. The bolt started out withdrawn into the frame and the nub at the front of the trigger stood proud of the frame, locking the cylinder by the front row of slots. Since the nub was physically part of the trigger, it was not spring loaded but was instead stood proud quite rigidly.
Pulling the trigger or cocking the hammer would rock the trigger back, withdrawing the nub from the forward locking notch so the cylinder could rotate. At the same time the spring loaded bolt would begin to rise, so it could do its job of locking the cylinder in position at the end of the stroke.
When the hammer fell, the bolt stayed up just like any other revolver, locking the cylinder in battery. But when the trigger was released, the bolt would withdraw and the nub at the front of the trigger would rise back up, transferring the cylinder locking duty from the bolt to the trigger nub. So the cylinder was never locked in place by both devices at one time, it was one, then the other.
The top of the trigger nub was completely rounded over so it could enter its locking slots without binding. There were no lead ins for these slots because the trigger nub was not spring loaded. It simply rose into the appropriate slot because the cylinder had already been positioned when the hammer went back. The long vertical cuts are clearance cuts to allow the gun to be closed if it was closed with the cylinder out of battery. Since the trigger nub was not spring loaded, it would interfere with the body of the cylinder if the gun were closed between slots and the clearance cuts were not present.
It was an unusual system. This was also the system used on the first two models of the much smaller 38 Double Action revolvers. By the Third Model of the 38 Double Action, S&W changed the locking mechanism and did away with the trigger locking nub and the vertical slots on the cylinder. But they never changed the mechanism on the larger 44, probably because there were far more of the smaller guns sold.
This 3rd Model 38 Double Action lacks the vertical clearance slots the earlier two models had.