Closing the loop on this. I think I solved the loose mag failure to feed problem.
On my African carbine (in another thread), the recoil plate was super loose - whole rifle jiggled in the stock. So I bought a type III for it - solved that problem. Just so happened the old recoil plate was an NPM type II. So I thought, what the hey, might as well try it in my failing-to-feed NPM, and replace the inland one it came with. It fit better than the one that was in it, and reduced the mag jiggles by half. So, I bought another NOS type III, fit is super tight, and now there is hardly any play in any of the magazines.
I think the type III recoil plate is pressing the top of the trigger housing forward, or better to say not letting it slip to the rear. You can see what I mean in this picture.
Only time at the range will confirm. If it starts to misfeed again, I have a new M-stamped mag catch to put in.