My understanding is that a lot of these systems are based on the same thing - IBM's TPF operating system where is is all about performing fast transactions.

Someone once told me that the locator actually maps/hashes to a disk location on the system. I don't know if that is still the case but I don't doubt it at least in the beginning of these systems. Just think these systems were developed 50+ years ago and memory, disk space, network and processing power were all expensive. So, they had to squeeze every byte and CPU cycle and wrote in assembly language (simple machine instructions).

I know on the UA SHARES system, if I make a few reservations in a row, the locators appear to be in some kind of sequential sorted order.