I have an SP101 back at Ruger right now to have the cylinder retention "button" replaced. It is rare for this piece to fall out, but here's what happened. The button goes in the recessed hole in the side of the frame. On the inside of the frame, the tip of the button is hollow like the shank end of a pop rivet. This end of the button is swaged or crimped into a couple of slots which are on the sides of the hole on the inside of the frame. Then Bubba at the factory takes a rotary grinder and smooths down the swaged end of the button so that it doesn't stick out and interfere with the trigger guard assembly which fits in next to the sides of the frame. If you look inside your old model SP101 the next time you have it apart, you will notice where some hand grinding has been done on the inside end of the button. Unfortunately, sometimes when Bubba comes to work he is a little bit hung over. And he grinds too much off of the swaged end of the button. So there is not enough material left to hold the button securely in the frame and it eventually falls out. Like the one I just sent back. And, of course, Ruger doesn't sell this part so you can't fix it yourself unless you machine a new replacement button. The new cast in nub, although not attractive, is a stronger setup and also a big cost savings for Ruger. How often does the "old style" cylinder retention button fall out? I've had two fall out over the last twenty years. So if you've got one of the old designs I wouldn't worry about it.
