Hi,
That is a nice pillion seat frame. This type is a post war civilian pillion seat, most likely produced by Acumen. It was never used on post war WD motorcycles. I am able/willing to make a cover for this type. I have the same frame (with 80% of an original cover) in my collection of pillion seats.
So sorry for my delay getting back to you as not been on here for a while. I am currently in Poland with work and back next week. I will chase as soon as back as hoped it would have been there by now. Abit annoying!
Feel free to email me which you should have done mate as see these regular.