I have one of Lex's filter elbows on my WD/CO/B. The pipe needs to be shortened for an Enfield, but it all fits fine. I have the earlier more bulbous tank on my WD/CO/B. I think later bikes had a flatter topped tank, maybe to take the filter better. I will send you a picture.
I just got my Elbow assembly today-Many thanks for all the work you put into making this assembly! It's way too cold in my garage now to put it on my M20, but I'm sure it will work well when I do have a chance to work on it. As a side note, I also got my Third edition of "British Forces Motorcycles" today. I already have the first and second editions.....