Reid, thanks for your detailed response to my post. I agree, "failed" was overstating the case, but BMC did so many experiments that never saw much of the light of day. Not much different to other large motor manufacturers, really.
John J, BN3/1 is a four (or two plus two) seater.
I have seen several references to the "economy" model Healey six as the BN5, but never any proof.
Personally, I have always doubted the existence of it. The Sprite, introduced in 1958 was designated AN5. The way the Austin Motor Co allocated model numbers is explained in most of the books: B indicates an engine of at least two litre capacity, N indicates a two seater and the numeric indicates the sequence. This formula was not strictly adhered to as the later models came along, but we do have a sort of logical sequence for a while: BN1, BN2, (BN3), BN4, AN5 (where A indicates a 1 litre motor), BN6.
I believe that the Jaguar C-Type started out as the Jaguar XK120C where the C stood for "Competition".