calling all bucket seat experts!

[halfstep]

FWIW
73+ brackets (nice looking seats BTW)
I just checked my (69-72 and 73+) buckets - your brackets look exactly like my 74 and not my 72.
In addition to the measurements, the indentations on the triangulated front support/riser are different. 69-72 have a symmetrical "U/V" shape and the 73+ have an off set "V". I'm attaching an image of the 69-72's I just sold. Take a look.
Like others have said, I think the seats could be any year from 69-77, but given the brackets more likely 73+ just re-done looking more like 71-72.
Certainly some seats look a bit more square, sharp, or narrow at the top and some have a gap between top and bottom, but I would guess that is more in the foam, cover, color and workmanship. I think the frames are the same. Once redone any of these years could be made to look like another.
As we all know, however, the brackets are distinctly different.
My observations only - others likely know better.