Those bolt heads are rusty/worn 9/16", since the bolt diameter is 3/8" on the clamp bolts. 3/8" bolts have 9/16" heads. If a 9/16" 6 point socket won't grab the wrench flats, try using a cheap 12 point 13MM socket that you don't mind damaging, and hammer it on. 13MM is slightly bigger than 1/2". Soak the threads with KROIL or some other penetrating oil, and/or heat them before wrenching.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!