Claybuster :
I have a friend that is a machineist and he made these in the
70's for guy's. He says that the original he thought came from a guy that still has an ad in the back of the skeet review "Thomas Morton " ? State MD U.S.
He advertizes shell catchers ect. My friend is a very active skeet referee at registered shoots in Canada and U.S. He say's that lots of that item were in use at that time but were plaqued with fouling. As you said the only real invasive part of it was the drilling of the 2 holes in the mag. tube and the cap.
That's about all I got for info.