EDIT: to clarify, all the pics I pasted were from 4-speed cars, that were claimed to be original 4-speed cars.
So, now that I've overloaded this thread with pictures, here are my thoughts:
I don't have enough coverage of years, engines, and pedigrees of originality to be conclusive.
A few of the cars above seem to be genuine non-modified examples (including the black car that started this thread).
Amongst them, some have it, some don't - which seems mildly-conclusive that there's not a hard/fast rule.
65 cars seem to NOT have the brace (383, 413hp, and 426hp cars are shown above).
I don't see any 66 cars with the brace. I don't have an original 66 440 car to review, though, only 383s.
67-68 cars SOMETIMES have the brace, and does not seem to matter whether 383 or 440.
It would seem like the 67-68 cars maybe had a HD clutch option (the 11" clutch housing?), and maybe that's what has the brace?
I also wondered if it had to do with MB vs PB, but they are both ways on that, too.