Did not realize that this was already partially answered.
Additionally they are grade 5. Fairly tall square shank, of 3/8" under the head. 3/8-16 Hex flange nuts, for attaching.
AMK lists B-12744, for the Mustang gas tank straps, but it is only 4.0 inches long, and has a shorter square shank under the round head. Page 8 in the 2014 fastener catalog.