Hello Manfred,
Both the 3 speed and 4 speed gearboxes were available on the T6. The easiest way to tell which box you have, is to check how the gearchange linkage connects to the box. The 3 speed box has a lever on the main casing whereas the 4 speed has a rod that moves vertically in the inner cover. If you have a gear missing, it is most likely the small gear on the inner end of the layshaft - they sometimes lose teeth after high mileage. If you are a member of the London Douglas Motorcycle Club, you could try the prewar spares dept - they used to stock that gear.
Hope this is some help,
Regards,
Eddie.