Bertrand, please excuse me for interrupting your puzzle. The Monoposto Formula is a UK national formula which originated in 1958 and was intended to encourage enthusiasts to built their own single-seat cars. There are many examples on Autopuzzles, like the Beagle and Terrapin. In order to maintain full entry lists the rules relaxed to include old single-seat cars from several formulae and various eligible engines.
It's almost like Formule Libre but monopostos only.