Postage and guaranteed delivery fee paid

Very interesting item, daniel, thanks for posting it.

First of all, to respond to your question, I have never seen this label before.  Second of all, I did a web search of the phrase "Postage and guaranteed delivery fee paid", and all the citations I found were for Royal Mail, but a bit oddly, none contained the actual phrasing on the label.  As near as I can tell, it seems that guaranteed delivery is a service that Royal Mail offers, normally for a deadline time, such as 9am the next morning, or something like that.  You probably know all of that already.

But a search for labels with this phrasing came up empty.  The silhouette of the Queen does not look like one I have seen before, either.  So, still a mystery as far as I am concerned, and the best I can offer is that now that I have seen your labels, if I ever see anything like them again someplace, I will try to learn more and report back.

Thanks for sharing!  Sorry I couldn't be of any real help....

Thanks for the follow up, daniel.  How were you able to figure that out?  Do you have a contact at Royal Mail?

I am not sure if I really understand what "internal use only" would mean in practice for a label.  Wouldn't the item with its label eventually be delivered to someone outside of Royal Mail, i.e. a postal customer?  Or by internal use, do you simply mean that only Royal Mail staff would have access to such labels and would apply them at their own discretion?

To me, their purpose would seem to be similar to that of an etiquette or auxiliary marking, just indicating that a particular service (like airmail, special delivery, etc.) had been paid for on a specific item.

"Guaranteed Delivery" puts it from 1993 onwards - the combination of 'Registered' and 'Special Delivery' - but i don't think it was many years before it was renamed the 'Special Delivery' we know nowadays.
In recent years for internal use Royal Mail have used their own envelopes, with a circle in a rectangle emblem top right, and a Special Delivery barcode sticker if necessary.   But for the delivery of stamps from their Swindon depot to Post Offices it's the usual PPI Special Delivery plastic envelopes.