It's a BSA Parabike, as detailed in the first of the links posted by Vintage_Cyclist above:
https://www.popularmechanics.com/mil...ding-bicycles/

There's an early twin seat tube example on the OldBike.eu website:
https://oldbike.eu/1942-ww2-bsa-airb...in-tube-model/

And here's the later much more common type with the single seat tube:
British BSA folding "Parabike"

I'd never heard of a Parabike until Pashley built a replica a few years ago - I came across it in the 'Lovely Bicycle!' blog - Harris Cyclery stocked it, probably still do, the only trouble being it doesn't fold as per the original, but it's still available. I saw one parked outside my local supermarket, the only Pashley I've ever seen 'in person' (as the writer of the aforementioned blog might put it!).
No Paratrooper Training Required
https://www.pashley.co.uk/bikes/bicycles/parabike.php

There were American equivalents - Columbia/Westfield 'Compax' and I think Huffman - but others on here might know more about these than me. I believe they continued to be produced post-war as folders.

It definitely is 650b, though