My old Ultimate had a square post for the backrest.
My old Ultimate was completely trashed (first few year production). I was nearly sitting on the battery box.
I bought a new seat at Asheville Inzane.
I thought to reuse the backrest, but could not with the square post. All Ultimate backrests have had the flat-stock posts for years and years.
I like the Mustang backrests better than than the Ultimate backrests, much better fit to my low back (at 5'9"). And all Mustang backrests have the same flat-stock posts that Ultimate does, and my Mustang backrest fits both my Bigboy and Lowboy (it was originally purchased for a Kawasaki Mean Streak).
IMHO, backrests do not wear out like seats, so keep your eyes out for used.
Ultimate backrest.

Mustang backrest. (not for Harley)
