The one listed for a WD9 is the same as an M and H.
I would measure the neck inner and outer diameter and depth. Find you a good autoparts store that knows how to use a book. They hopefully still have a book for radiator caps with dimensions. I had to do that for my old michigan loader. The parts guy made it seem like there was only a few sizes mulitiplied by different pressure ratings.
Second guess would be to remove it take it to a readiator shop where they can remove the neck and install a new neck of a known size for a cap.