For whatever reason, all the "tall tail" versions but the D models were retired by the mid-70's, leaving just D's, G's and H's on active duty. The B-52F's were stationed at various Air Force Bases, including closed ones in remote locations, just to keep the Soviets nervous. But the F models had seats that for some reason were considered to be preferable than the D model seats. So the D's and F's had their seats swapped.
You know, it is a measure of just how unjust things can be that the design team for the B-52 never won the Nobel Peace Prize.