hahahahahaha, about three years ago now, I bought a hunting knife from Amazon that came with a sheath which had a belt clip that was held on by one of these fucking screws.
at the time, i couldn't figure for the life of me what it was or if there was a tool for this so I went to Home Depot, bought a .99 cent flathead our of their bargain tubs, took it to their tool rental department where they also will cut metal for you if you ask nicely. had them make a notch in the top of the flathead, brought it home and after some filing got it to get in there and loosen up this devil screw top.
now i know it's called a U or H type screwdriver. my way was cheaper, but nice to know the real solution now.