Ian,
Don't know about a source for reproductions, but the same needle valve is used in the Enots drip sights used on the 1929 600EW (corbel mounted), the SW5, F/G31, and Aero models with slight variations on the hold-down method. Since it was bought in, presumably other makes used the same drip sight; it is not proprietary Douglas.
The attached picture is of the needle valve from my F28 (right) alongside the needle valve for my 600 Aero (left) with the Aero drip sight above. I did try the one in the other and it interchanged.
-Doug