The gearbox is equipped with a level-plug (RHS, just above the drain plug) but its access is abyssmal, being masked by the footbrake pedal and the clutch arm.
Fortuitously an alternative approach is possible.
The LHS rear corner of the gearbox inspection plate was originally secured by a 1/4" BSF stud and nut: this, or a non-original 1/4" BSF bolt can be removed to reveal a drilled-through hole, which permits insertion of a thin and slightly flexible oil dipstick! A piece of 16 swg (dia. 1/16") piano wire is suitable for this.
Going from the original level plug, oil-depth should be about 1"
A warning, Remove Dipstick before Starting Engine!
