Hi Pete, We have an 1947 B33 which also had a later box fitted with the same one side cut out as your picture. Have not checked if that cutout was correct regarding the chainline but replaced it with a correct early box.
They are nice little lightweight bikes but I agree that the early box is probably to light for the 500.
For me, cutting down the feet of the case would be out of the question.....you also have to remove the ribs which supports the feet. If I really wanted it I would first concentrate on modifying the plates in a way so they can take the later box.
thanks Ian, I think I have a spare case in the parts bin but I'll keep the offer in mind if I can't find it. I hope to get to Torrington as soon as this virus allows!
I am a bit concerned about the strength of the case if the webs are cut off but thought I'd go that route before making new plates. What's the worst that can happen? :thinking_face: :fearful: :scream: