This could well turn out to be a black-hole candidate. I have checked photo records for Le Mans - in 1949 there was a Simca-engined special entered by Ecurie Verte for Gay/Baboin which had a modern-style barquette body, but the wheel-arches are the wrong shape, and it was powered by a 569cc engine. The whole body swung up for engine access, hinged at the rear.
A car which had a similar body to the "mystery" car was in the 1950 race but was Renault 4CV powered. Built and raced by Pierre Ferry it had the engine at the front.
Anyone got pics of French small sports-racers in other races of the period?