Jeez. It's a cobble-up, and a badly-done one at that. I wouldn't trust to ride it - somebody has cut and shut the subframe and made a terrible mess made of the tube just aft of the swingarm mount plate. J2 front frame, sidecar yokes on the forks (look at the space between the headstock and the stanchions), late-type Enfield India swingarm with the reversed axle plates.
The swingarm military 350 in May's book (page 110) I think you are referring to is based on the factory Model G de luxe, aka 350 Clipper. This had a Model G donk and tranny in a modified mid-50's swingarm frame - note special casting lug for mounting the swingarm. Roy Bacon
's book Royal Enfield - The Postwar Models has a pic of a properly done swingarm J2 captioned not listed as standard. This is clearly in a slightly different version of the same frame - you can see the swingarm lug quite clearly. I'm only guessing, but I'd reckon it was only an experiment - since the 500 Bullet was already available, what was the market?