Wensleydale
Definition is in APP-6 (which is on line at
http://armawiki.zumorc.de/files/NATO/APP-6%28A%29.pdf
Zombie – A suspect air track conforming to ATC rules or NATO airspace following a recognised traffic pattern. (STANAG 1241).
No idea when STANAG 1241 was first issued, but the first RAF radar to have synthetic track labeling on the PPI was the Radar Type 82 as used for Bloodhound Mark 1 Tactical Control and Z wasn't one of the labels available on the prototype system at North Coates in 1958/59. The lables on the system were E, F, H, I, IF, M, U and X (The AP text only lists a few labels as F = Friendly, U = Unidentified and H = Hostile). I suspect the change to a common NATO standard as regards AD stuff happened around June 1960 because that's when the Bloodhound's designation changed from SAGW to SAM as part of a move to use NATO nomenclature as a UK standard.