Royal Enfield Bullet 4-Speed Gearbox 

Getting into the Bullet Albion gearbox
NB: DETENTE PLUNGER (top right):
Unlike that shown in the video, its slot should be HORIZONTAL, to match its inside, which is like a sturdy screwdriver tip.
(Though in this case, the plunger had come apart, so it MAY have been the situation, that while the slot (in the video) is vertical, the tip at the other end was horizontal.)