OK, checked the car this morning, and the 2 bolts that attach each of the inner VE3 brackets to the bumper itself have 'RBW' head markings (as do the bolts for the outer brackets). The bolts that attach the bracket to the frame horn/extension have 'L5' on the head. Anyone seen this head marking in this application before?
(FWIW, the car had a starburst drag racing paint scheme applied over the original silver in 1971, you can see the paint overspray on the bumper and brackets, but it is evident on inspection that the front end was painted without being disassembled, so I have no reason to doubt these bolts were on the car, at least in '71, if not when it left the factory. When I eventually remove the bumper (and brackets) for repair, I'll take photos showing the length of these bolts.
'RBW' (bumper to bracket):
'L5' (bracket to frame horn extension) sorry about photo, not very clear but hard to access: