Lots of references here Greg, and here's yet another one. Kick-Killer leather slip ons. If your after recoil reduction without permanently altering the butt of the gun, this is the way to go. They come in numerous sizes. I have them all. I probably have 6 of these and have never had an issue with them.
I use them for LOP increase. They add 1/2" to 9/16" and come with an either Velcro securing flap, or leather laces. Super good quality, they stay secure in any conditions, and importantly (to me) Made In America.
https://www.chuckhawks.com/kick_killer_recoil_pad.htm
Threaded Mode