That's it. You guessed right. The ST 61 (I have 4 of them) are the only engines that I have that show these pits. The two S61K's that I have were acquired used and were both pitted.

You should get your ring from Frank Bowman. You could let him install it maybe? You should ask him? There has been too many variations in them from Super Tiger as to roundness and ring gap. This is a pinned ring. Usuallly the pin is at the left front but if the piston is in backwards, it would be in the right rear.

There may be some other damage if it was run that lean that long? Look to see if the piston is scored on the sides. Look at the inside of the sleeve for scratches? Where that ring broke, there should be big scratch on the sleeve. You will have to get a brake cylinder hone to clear this up.

Good luck

Jim