This most of the time a custom affair..the shoulder typer recoil plug was built for comp guns not using a bushing..the last couple I had converting old comp guns to 5" guns needed have them custom machined...I have never seen one offered..
I am sure George could machine one given he has time..give him a call..I bet Chuck Rogers can do it too from seeing his work here on the forum..
Clark made these for years. I just gave DonW one with no hole cut for a Caspian slide. You can get to do or Jim Clarks outfit.I'm sure Don would love to part with it and drill the hole size you need in it.