This is dual marked 18 and 21mm, also 3/8 and 7/16" on the opposite side. I am not sure if that means whitworth or USS. The only other markings are "EHLI(?) DEP." Actual sizes are closer to 3/4" and 7/8" .
Does anyone here know anything about this wrench? I bought it because of condition and cool factor, plus it is still usable.