Interesting. I haven't seen a subframe supplied by Jawa that before. I have a feeling that it is not an official Velorex product but something that American Jawa (US importer of Velorex sidecars) had made.
It looks a lot like part of the subframe that Side Strider used to make but without the part that runs back to the rear engine mount. I think I would want to add that part if I was going to use it.