So lets say I pick up one of these Yugo milled underfolder kits from Centerfire systems.
yugo kit from centerfire

I know the front trunnion is set up for the thicker milled receiver, but my question is about the underfolder itself.
Would it fit in a stamped receiver no problem?

Also, if at some future point, could you remove this barrel and put it in a front trunnion for a stamped receiver with no extra fitting?

Anyone know?

IIRC, the outside width of the milled receiver is the same as the outside width of the stamped reciever, so yes, I think the underfolder mechanism will work on a stamped reciever.  I am on the road now, but if no one verifies I am right, I can double check my memory when I get home.  But to use the UF on a stamped reciever, you will need to get a UF trunnion to fit inside the stamped reciever.  It would probably be possible to make a trunnion by grinding down the milled reciever stub to the right thickness.  Regarding the barrel, good luck getting one in good condition.