I'm going out on a limb here but it's possible that the OD switch and the reverse light switch are the same. The parts book only shows one on the gearbox and the electrical section refers the back-up light switch back to the Gearbox section without giving a number. The part number of the reverse switch is in the index as #662292, says 2 off so that's encouraging. My spare GB/OD a few miles away. You'll be able to see when you have both units out of the car. You should then have 2 undamaged switches for installing the B20/M41. The switches are just switches and don't actually physically lock out anything in the GB. When in top or reverse, a part of the GB selector mechanism pushes up the switch plunger. When in any other gears the plunger is "down" and the switch is "open".

Good luck.
Derek UK