No. 63b Seaplane 'Mercury', the mail aircraft that was carried by no. 63a Flying Boat 'Maia', together forming the set 63 Mayo Composite Aircraft (1939-1941), had the correct registration number G-ADHJ (see picture). The same no. 63b was re-issued post-war as ‘Seaplane’, without ‘Mercury’ transfers on sides below the cockpit, without gliding game hole on top and without clamp underneath, for coupling with 'Maia'. And, it had a new, but fictive number G-AVKW. 

Just curious if there was any chance that this new number was ever in use for a registered aircraft, the internet gave a quick answer: yes there was! The number G-AVKW was assigned to a Belgian aircraft, a two seater double decker Stampe-Vertongen SV-4B C/N 561, developed pre-war, but this version built in license in France by SNCAN (Société Nationale de Constructions Aéronautiques du Nord) in 1947. It was registered two decades later in the UK: 1-5-1967 (these were very popular second hand aircraft for recreation, stunt teams etc.). It didn’t last long, however, after that, because it crashed near Godstone, Surrey, 29-10-1967. Registration belatedly cancelled by the CAA as aircraft ‘destroyed’ on 29-6-1970.

See also: http://aviation-safety.net/wikibase/wiki.php?id=65788 Kind regards, Jan