If you have a mouse with the small round PS/2 connector that you would like to connect to a 9-pin COM port, this is the solution. Plug the adapter into the COM port of the computer, then connect the mouse cable to the back of the adapter. Note, however, that not all PS/2 mice work in a COM port only mice designed for it will function through the adapter.
Sandberg Adapter PS/2-mouse on 9p COM. Gender: Female/Female, Connector(s): PS/2-mouse, 9p COM, Adapter type: PS/2. Color of product: White. Package width: 0.335" (8.5 mm), Package depth: 0.0787" (2 mm), Package height: 0.492" (12.5 mm)