I'm trying to restore an old park bench, and I am struggling to unscrew the bolts. The heads have this wide (~12-14 mm) "flat cross" pattern that I have never seen before (see photo), and I don't have any screwdriver that nearly wide enough to get any kind of grip on them.
My googling attempts to find the right tool have been futile. It might be because I don't know exactly what to search for, but common screwdriver bit sets don't seem to have anything approaching that kind of size.
What would be the right tool to unscrew these and where could I find it?
And, for what it's worth, square nuts at the back:
Drilling successful! One down, 23 to go...