According to David Hodges' book there weren't that many single seater U2s. Could this be the Mk 12 Formula 3 car?
Not that. Much closer to "20" and for a formula associated with a famous marque. Still locked.
(PS I know the book of which you write - I think it restricts the content to F1/F2/F3 and possibly FAtlantic/B or else it would have to be a lot larger. This is none of those formulae. Much more like a national formula.)