They are known as PPI (Printed Postage Impression).
First introduced in the 1960's to help bulk postage users save time and money by printing their own stamps on labels, envelopes, etc.
Post office regulations stipulated that the inscription on the label must include the words 'POSTAGE PAID', the user's unique serial number (e.g. PHQ 933) and a service indicator.
Service indicators used were 1 (inland first class, overseas mail and surface), 2 (inland second class, overseas surface printed papers), R (inland second class rebate), P (inland and overseas parcels). Later composite indicators, P2 and RP were introduced, but don't ask me what they mean...
PPI users initially could only use one office for postings, but this was changed and those who used more than one office bear markings such as yours with the authorisation from Postal Headquarters (PHQ). The PHQ designs intended for overseas mail had to include the words 'GREAT BRITAIN'.
So from all this we can deduce that your PPI's were produced for overseas mail.
The identity of '993', however, eludes me.
I know that '2204' was the BBC, but that doesn't help you.
There is information here about how the PPIs work today
http://www2.royalmail.com/discounts.../ppi/licence
Posted 10/13/2011 8:44 pm
) but I am interested to hear about these guys. I bet Gavin or LB1 will know..... 
