Quoted:
Pics?

I think these had a tang welded to the rear of the under folder receiver which has a flat backing. Therefore it won't take a standard fixed stock as the rear of the receiver is covered.

I've never converted one but am familiar with these as I keep an eye out for them. I was under the impression if you remove the rivet that is covering the under folder stock hole, a Chinese under folding stock will drop in. They pop up on gunbroker with the assembly. Some surplus sites have the stock frame itself.

Then cut off the tang that was welded on and refinish.

Again, I've never done one but this is what I have seen others do to convert them.