Firstly there are the unstamped ones which enable users to pay by stamps, by franking machine or by Horizon label at the counter:
Another type is used by companies which send a lot of special delivery mail. This is an old one from Crucial Technology which has the Postage Paid Impression on one side but the bar-coded special delivery label is on the reverse:
A fourth type is the 'Business Response Special Delivery' - only recently discovered by me but they may have existed for some time. The two examples I have found are used by a company offering to buy your surplus gold. Two sizes have been found so far:
Thanks Harry for showing me the first type with a Horizon label paying the fee; I should have included that as an option.
Thanks also to Paul D for sending this further example of a PGFC bag (note address now Poole rather than Bournemouth!) This has the same sort of Royal Mail bar-code label but the only indication that this is to be sent by Special Delivery is in the top panel which reads
Special Delivery response
Licence no. NAT247817/PL1
The whole style is very odd. Are there any others out there, in different colours? Any more which just have the word ' Mail ' at top left, and no Royal Mail SD branding? Paul says that "the reverse states: 'This envelope will be sent via Royal Mail Special Delivery' (Again not in red nor Royal mail font)."
Do please let us know, using the comment link below.