I had to modify a 742 magazine to fit my old 760 game-master. It works, but barely because the spring in it is so weak. It holds 2 shells and functions 80% of the time.
I was thinking about trying to have someone 3-D print one because they are so hard to find...IF you do find one that works with an older 760 they are $150....and thats a big IF on finding one.