It was a requirement for all racing cars to be registered at least during the early '70's, something to do with public liability insurance I believe. At least for single seaters the usual category we used was "E class A" which was the same as a farm tractor, thus avoiding the various WOF requirements. Somewhere there will be a record of all those registrations and the make.

The term "Homebuilt" is a generic term now preferred by LTSA, I run trucks for a living and most of our heavy trailers are registered new as Homebuilt despite being manufactured by reputable trailer builders.