Not sure if they were the same in the UK or Canada, but we had Reverse Lamps in our Mini Clubman Lamp assemblies, which were the same in appearance and the main body as the MKII UK types. These we had from January 1971.
For the Reverse Lamps, they had a double filament bulb in the Indicator socket, however, each filament was rated 21 watts, an unusual bulb, but still available. One of these was for the Indicator and the other for the Reverse lamp.
In regards to the shifter, I would think it would have had two separate switches fitted to it, one being a Reverse Lamp Switch and the other likely to be either a Top Gear or Neutral Switch. My best guess here is that it is in fact a Neutral Switch, (which is Open when in Neutral, but closed in all other positions). So that when in any gear, and if a front seat is occupied, the seat belt warning will Indicate unless the seat belt is clipped up.