HSM Formula Junior?
We have a winner. This is indeed an HSM FJ, created from a Formula Vee front end and a reversed DKW FWD front end at the back. Hence the amateur-looking radiator installed at the front.
Three of these cars raced in the December 1959 FJ race at Sebring which was won by Walt Hansgen's Stanguellini. The puzzle car was driven by Harry Baumann but I thought it was the car of Ray Heppenstall, who famously raced the Howmet TX turbine cars in endurance events.
Well done.