This is a good question and one that I didn't manage to find a definitive answer to when I was trying to pin down the ID of WD frames. I'm a little wary of trusting the re-touched images in the Norton parts books. I have 1937 and 1938 copies and the illustrations are identical.
Are you referring to the 'open diamond' or cradle frames ?
I have a photo of a 50000 odd numbered pre-war diamond frame which should be 1932 and it has a curved tube.
I know of a 1938 Model 19 and that has a straight tube diamond frame.
Post-war rigid cradle frames seem to have curved top tubes.
Not a lot of help, I'm afraid.
