An earlier mouse that plugged into an expansion card in the PC. In the 1980s, most mice used a serial connection; however, a bus mouse was occasionally used to free up a serial port for another peripheral. The bus mouse connector looked like a PS/2 connector, but the pins were not the same. See mouse
and PS/2 port
Bus Mouse and PS/2 Looked Alike
The outer shell of the plugs was the same, but the pins were different.