It is an Allen (hexagonal) key. It is an imperial (inch) size key of 5/64. Metric people do not dispair. Five sixty fourth inches (5/64) equals 1.98 mm, and a 2.0 mm Allen key will fit perfectly.
I note that the screws in later production equipment (they are all the same in the F-1X, F-2B, SF-2 and DS-5R) are mede of softer steel than those in older gear, and if you are not careful, after a few loosening and tightening of the screw it is easy to strip the hexagonal socket. It can then be difficult to remove the screw. It can also be difficult to source a replacement, although I am sure Alembic would help you out.
Cheers,
Peter Jonas