Petty-JAP, 900cc ?
The Petty on "that site" has twin-wishbone rear supension like the puzzle car, which must have been pretty adavnced for F3 in the 50s. In fact, they look fairly similar, and possibly a hillclimb specialist has doctored the original car after its usefulness on the circuits was over.
(I realised I was wasting my time on "that site" when I eventually spotted two exhaust pipes, which precluded the obvious engine size)