This is a nice clean double-ended speed wrench that is marked "CYLINDER HEAD SCREW" on one side and "CONN. ROD NUT-90655" on the other (the numbers are a bit hard to read). This may have been intended to be a 3/4" wrench, but it does measure 13/16 at both ends, so it will fit the larger 13/16 hex nut. An interesting engine wrench.