Those are "clutch head" screws and the appropriate drivers are available through the various parts vendors and even at the local NAPA-at least here.
4-piece set at LMC Truck
http://www.lmctruck.com/icatalog/ca/full.aspx?Page=31
Individual driver at NAPA
https://napaonline.com/Catalog/Catal...006432837&An=0
Just for a couple examples.