Some later models supposedly had a switch on the cover but my m40 and m41 have a ground lug on the little "doghouse" cover over the rear ends of the selector shafts (thats the "back" side of the tranny, sicko). It goes to a spring inside that the reverse selector touches when you go into reverse ('bout as simple as it gets!).

The OD switch is actually a 4th gear "enable." Some other cars that use the Laycock OD have 3rd and 4th gear "enable." The issue is that the torque from (yes even our little bangers) the engine in the lower gears can overwhelm the cone clutch that engages the OD planetary in the unit. You are also not supposed to shift into OD while accelerating or (you guessed it) more whelms.

The REALLY important reason to have the switch is it prevents (in most cases) attempts to drive the car in reverse with the OD engaged. Going backward in OD causes the guts to get jammed and disassembly is required to fix it (if it remains fixable!). The OD unit is strictly one-way (even the whelms).

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Mike!