"Ruediger LANDMANN" wrote in message
...
Been browsing around and can't find an answer...

Is the first part of the designation the letter "O" or the digit "0"? What
does it mean?

O/400 is simply the Handley-Page type designation and yes, it is an O
('Oh'), not 0 (nought). The H.P. designation letters eventually went up to
'X4b' (however, there were a few X-bomber projects in the 1920's-'30's),
although there was some overlap with this alpha/numeric series and the
later, more familiar H.P-whatever number dsignations (for e.g. H.P.53, the
Hampden). The X4b was a modified D.H.9a, one of the many aircraft used for
leading-edge slat trials. A comprehensive HAndley-Page history is: Barnes,
C. H. (1987) (2nd ed.) Handley Page Aircraft Since 1907. Putnam Aeronoutical
Books, London.