In Beags day (and mine) we had 'In Use Reserves' Aircraft that had no authorised scaling, fuel, stores etc but which we could use but 'robbing' labour and fuel from the rest of the fleet.
Bomber Command also had a fairly large reserve of aircraft squirreled away.
In 1964 we had 3xMk 1 Vulcan Sqns, 3xMk 2 Vulcan FF and 3xMk 2 Vulcan BS and the last Mk2 (ex-Skybolt) had been delivered. Then the Mk 1s were replaced with Mk 2s and the MK2BS disappeared and the only slack cut in Strike Command was the disbandment of 12 Sqn. In other words 5 Sqns of Mk 2 ac came from the Command spares holding!