Hi,
Recently I found this cover:

The cover was mailed from Dubai to London, England on 11 NOV 1972. It was franked with a 5dh Shaikh Rashid, underpaying both the airmail and surface rates to England.
One of the markings is "Insufficient Postage For / Transmission by Air Mail". The cover also includes three 3p postage due stamps.
Note: I did not see a general postage due forum. Perhaps I am mistaken?
Best wishes,
AndrewG
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