The PS/2 connector is what came after that the bus mouse. Back then the mouse was connected to a specialised add-in card. Probably an ISA card if I remember correctly.
tldr: Ball mouse containing only optical encoders and no other electronics. Very popular in late eighties. Atari ST, Amiga 500, Acorn Archimedes (the ARM computer) all used this type of mice because of price.
Bus Mouse controller card used special Alps Electric 64H101 quadrature decoder and counter chip converting raw encoder outputs to neat X Y coordinates computer can read in just a couple of IO operations, much easier and quicker than serial or PS2 mouse.
https://en.wikipedia.org/wiki/Bus_mouse https://en.wikipedia.org/wiki/Industry_Standard_Architecture