10-Pin IDC header in Lyon mouse-
When facing the connector from the rear of the mouse, with the mouse right side up, pin 1 will be on the right.

My Lyon had MS3 snipped internally. It came with a DB15 connector on it for an unknown machine.

IDC in Mouse || Dest on 2DE19P (Alto) || Signal Name || Desc

1               16                       MS2            Right Mouse Button
2               2                        MX2            X Axis Gray Code
3               1                        MX1            X Axis Gray Code
4               4                        MY2            Y Axis Gray Code
5               3                        MY1            Y Axis Gray Code
6               15                       MS3            Middle Mouse Button
7               19                       GND            DC Ground
8               -                        NC             Maybe another ground?
9               14                       MS1            Left Mouse Button
10              6                        +5             VDD