As illogical as it seems, I believe that size is 3/4” x 15/16”. The second half of the part number is supposed to correspond to the Industry Standard Number, as far as I can tell, and I don’t see a 731C in Alloy Artifact’s chart. I have some Double Grip DOE wrenches but I’m not sure of the sizes and I’m not where they are at the moment.
-Don