Post-war, I never heard the famous car being referred to using “Monoposto” rather than “Single seat”. Furthermore Herb Gilroy was the only one I recall mentioning “Rubber Duck” and that was during a discussion concerning its handling features. In general discussion post-war it was usually referred to correctly as “The Brooklands Austin Seven”.

Photographs I have turned up show that the car displayed registration plates:-
272.940 (Herb Gilroy) and 216-775 (Ron Roycroft)

I hope that this is of some help.