Simple flapper valves like this, using the weight of the flapper disc rather than a spring, are found on many veteran motorcycles - for example those used on JAP motors were essentially identical to the Douglas item. They were often found at the entry point to the crankcase, or on the outlet end of the drip-feed oil pump, or sometimes in both places.
If the disc is missing a piece of 0.005" brass shim (or a piece of aluminium Coke can when you've lost the original in the grass on the side of the road) cut out with scissors usually works fine.
Leon