A reader has sent a copy of a Special Delivery PPI (Postage Paid Impression) label, used by Courier/Distribution company FRT Group of Crawley last October.
I presume the company is allocated a batch of numbers and prints the labels when sending out tickets for events or journeys (note the 'Booking Reference').
This is on a standard DL window envelope, so I suppose there would be no excuse for not obtaining a signature on delivery. But without the sliver and blue of the standard labels, it would be easy to overlook this type of thing in the sorting office, given that so much mail has so many different non-postal labels, barcodes etc on.
Updated March: Another reader has sent a copy of a similar Recorded Signed For label:
It seemed appropriate to include it here. As mentioned in the 'comments' it seems even more likely that this would be overlooked, than the separately handled Special Delivery packet.