Francis Barnett used an alphabetical frame identifier system in the 1930's. A for 1930. B for 1931, C for 1932 and so on. This leads up to K for 1940, the last year of production before A Hitler stopped play, so I would expect your machine to be a 1940 model.
A more precise identification as to exactly what 1940 model you have would require the engine number also.
Hope to hear from you soon.
Cheers,
