You've definitely found the oil fill point - it really is that unsecured little rubber bung right behind the tacho drive so you can't get a decent funnel anywhere near it - wonderful DDR design
As for capacity - if you put 500ml in and doesn't come pouring out the check screw, either:
1 - it was completely empty and the gearbox was dry. Add a bit more and check if you can get it to overflow (I've had to fill my TS with about 800ml before and she was fine with it - makes less noise and the ride is smoother, but it's probably taxing the gears a bit...)
2 - The clutch-side gasket is dead and you're leaking a lot of gear oil. Try tilting the bike a bit and checking around the side-plate.
3 - The chain-side seals are gone and you're leaking gear oil into the space around the dynamo/chain - take the points cover off and check if the dynamo is covered in goop or there's oil nearby.
4 - If you've filled it, run it and then tried to drain it, maybe you're crankseals are shot and you're burning gear-oil. Check if your silencer/piston crown/spark plug are covered in oily black gunk. If they are, maybe time for some new seals.
5 - You've got a _serious_ crank-seal leak and your crankcase is now full of oil - take the head and cylinder off, and check that the crank isn't full of oil. If it is, DON'T TRY AND RUN IT, YOU'LL BUCKLE/SMASH YOUR CON-ROD.
Hopefully it's (1) and maybe (4) - but wouldn't hurt to check the others, just in case.
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