Hi Neil,
After a brief moment searching for the lost tin of oil fittings... I came up with two different non-return valves. The short one screws into a 7/16-20 thread, the long one has a much smaller thread, which seems to be tapered - something like 24 tpi on a diameter of about 0.300".
The long one is certainly DOuglas, as it is stamped with the DK monogram.
Is one of these correct for the EW?
When used on a vintage JAP engine, where oiling relies on crankcase vacuum, there is usually no spring on the flapper valve, but I think Douglas tended to use a very light spring to hold the flapper valve up against its seat.
Leon