[Rick Parkington]

Hiya, not a great deal of help I'm afraid but from memory the '37 Model 14 I had in the family didn't have this rocker box plug but I do remember a few odd 2 BA- ish screws that blanked off drillings where oil ways turned corners so maybe it started as one of those.
Can't recall the drain plug thread either I'm afraid but charging wise, the best tip I was ever given with dynamos is to energise the field when testing. This means fire up the bike with the dynamo connected and take a wire from the live side of the battery and poke it into the F terminal of the dynamo along with the wire already there. If nothing happens then the dynamo is faulty. If it starts charging, odds are the dynamo is ok. Leave the wire in there for a little while and then remove it. If it continues to charge you're sorted - although you may have to do it again. The residual magnetism in the dynamo that kicks off the initial charge sometimes dies away when bikes are left standing for a long time and this 'hotwiring' revives it while also polarising the field correctly to match the battery.
If it stops charging when you remove the live wire,  try again but if it steadfastly refuses to work unless the wire is connected, it is probably a CVC (regulator box) fault.
Hope this helps
Cheers Rick   

[72degrees]

Now I have the  '37 250 Sports running well after TLC to the carb (a 275 which needed a new float and needle, plus new throttle clip after a pingfskit moment). a couple of questions.

There is a 'plug' on top of the rocker box (see photo). Looks like a modern socket head screw of the wrong thread. Should there be a screw in plug and if so what size/thread. This one has a rubber seal but definitely won't screw down on to it.

What size and thread should the front chain case drain plug be? Another wrong set screw has been used temporarily. Part number 6787 with washer I think.

Still not diagnosed the lack of charging, but now I can start it easily and it idles well I shall poke about with my multimeter.