Some contributors have advocated sealing the end face of the dynamo against the crankcase. Don't do it! The Primary Chaincase needs to breathe. Ask Hugo Myatt about the consequences of applying sealant.
What happened to Hugo's bike was written up in MPH a long while ago. He was puzzled by the fact that his gearbox was getting pressurised, thinking at one stage that there might be a blow-through from the flywheel section of the crankcase. In fact, sealing the dynamo prevented the chaincase from breathing so pressure coming through the drive-side main bearings was being forced past the oil seal on the clutch shaft which, after all, is meant to keep oil in, not out. The method used by Hugo to confirm gearbox pressurisation is best left to him to describe.