James Boothby Motors made the JBM cars using reconditioned parts to avoid purchase tax, including Ford V8 engines. Might they have made one with a Lincoln/Mercury V12 which would explain the six exhaust pipes?
The April 1948 issue of Motor Sport contained a short summary of the JBM cars after Bill Boddy had visited their premises. He tested a couple of cars, both of which were fitted with V8 engines, and also summarised other products which they intended to build. No mention was made of any other style of motor, except a proposed single-seater, suitable for F1, which was to be powered by a Grey Fireball marine engine (!).
(I have just found out that the Gray (correct US spelling) Fireball was available as a straight-6. Just a hunch, but what about if Boothby gave up on his single-seater dream, and installed the marine engine in this sports car? Hence the homemade style of the exhaust pipes. It's just a thought..........)