Very likely that it uses an adaptor box. Almost all Macs (exclusive of the Plus) with
MiniDIN-8 serial also had MiniDIN-4 ADB interface, which would be what a mouse would use.
Even on a Plus I don't see why anyone would go to the trouble of rewriting enough
drivers to use the serial ports for a mouse when the mouse port is available and SSW
expects its use.
Could this be a NeXT trackball?