Update: This is a somewhat obscure security bit that goes by a few different names. On the iFixit website, this bit goes by the name "Spanner". iFixit has 4mm, 6mm, and 8mm versions for their 4mm screwdriver handle included in most kits available in the dropdown list [product|IF145-373-50|here|new_window=true], as well as 4mm, 6mm, and 8mm bits for the 1/4" screwdriver handle included in the Mahi and Manta sets available in the dropdown list [product|IF145-356-42|here|new_window=true]