Not only the top, also the lower braces were available on the 1973: Maybe it had to do with the heavy 5mph impact bumpers, but then again: they were chassis mounted. No firewall mounting points on the 1971/1972 models:
Some Boattail Rivieras also came with two chassis brackets. In this pic the new exhaust is in the way:
I think the 5mph bumper is the best theory. Even though they’re frame mounted, they’re a lot of weight way out there; they can exert a lot or torsional force on the rails that are the floor of the sheetmetal box. Patrick
When I had mine I was just under the impression it was a Buick upper scale quality thing? More bracing, more solid? Now I am thinking the lower bracing is for that huge and heavy US mandated 5 mph bumper??
