This is somewhat mysterious. Sabre launched in 1960 on a pair of of IBM 7090s (a system itself launched only a year prior) but the first HDD compatible with the

This is somewhat mysterious. Sabre launched in 1960 on a pair of of IBM 7090s (a system itself launched only a year prior) but the first HDD compatible with the 7090 was as far as I can tell, the well-known IBM 1301 which didn’t ship until 1962. Perhaps the record size was designed based on the anticipated specs of the 1301? Or perhaps they wrote a new storage layer when they got disks - either way I’m sure as you say they optimized their record size for those very expensive disks.