Is this it? I fancied the formation sign could be a 4th Armd Bde rat and that seems to be confirmed by the IWM's description. If it is 4 Armd Bde HQ, the unit serial should be 121 which, again, an eye of faith can see. The only snag is that the AoS flash looks to be 'dark over light', which is the wrong way round for HQ signals? The other possibility would be their Forward Delivery Squadron which had green over blue. No serial number is given in the resource at hand, but it's possible they could have used the Bd HQ number.