BSA gearboxes for rigid and plunger models use 4 screws to retain the gearbox inspection plate to the main gearbox case...
These have a head that is both hexagonal and slotted to enable the use of a spanner or screwdriver for removal and replacement...
Swinging arm gearboxes still use three of these screws but the fourth one is replaced by a plain slotted screw...
Access to the plate is virtually impossible with the gearbox in the frame, the inspection plate being used during gearbox assembly when the box is out of the bike...
I've thought about this and can't really come up with a good reason for this change...Does anyone know the answer..?..or have a sensible theory?...Ian