The factory Mini14 folding stock looks "sort of" like an underfolder, though it folds to the side. The Mini14 actually was in U.S. inventory for a while. Marine guards at Embassies had them. Saw photos of them in the arms rooms, etc. No, I don't know why they had them either. I think DSS also had them. I suppose that it is possible that if a Naval Special Warfare team was training with embassy folks one might have been used. Or it could have been used for training since a wide variety of domestic and foreign small arms were, and are, fired for training and familiarization.
Supposedly a few may have been used in South and Central America during the 1980s - commercial availability making them deniable presumably.