Oil can get through the left crank seal and fill up the primary.
Engine oil draining down from the tank and filling up the crankcase was my first thought too but, on a '69, that has different oil in the primary and so should be sealed off from the crankcase, I did wonder if the engine oil should be able to work its way through the drive-side main bearing and seal to fill up the primary? If that's what's happened, BSA didn't intend the alternator cover to be this much of an oil seal, hence the incontinence.
Sounds like it.
should I drain all the oil
. If you put different oils in tank and primary, they've contaminated each other in the primary.
. Even if you put the same oil in tank and primary,
there's likely to have been bits of odd debris in the bottom of the chaincase. Hardly worth chancing your engine for the cost of a few pints of oil?