The PS/2 connector was just a mini version of DIN connectors from the mid 1900s. They were sometimes used for printers and floppy drives in place of wider parallel connectors. In general, once you knew where ‘up’ was you could figure out the orientation in a way that is still more intuitive than USB. XLR cables are similar and still widely used, but have more external cues as to their orientation.

https://en.m.wikipedia.org/wiki/DIN_connector