Ceylon: Short Paid Aerogramme to the UK

Aerogramme sent from Colombo (25 NO 59) to London, UK. The aerogramme was pre-paid 40 c, which is 10 c short of the 50 c aerogramme rate. Circular “T centimes” handstamp (endorsed “13”) applied at Colombo. Upon arrival in the UK, a “3 ½ d TO PAY F.S.” handstamp in green was applied. Two postage due stamps with a total face value of 3 ½ d were affixed, documenting that the postage due was collected from the consignee.

Postage due calculation in country of origin

The 1957 Ottawa UPU congress (effective April 1, 1959) fixed the foreign letter rate at 25 UPU centimes (ctms). Member countries, however, were allowed to increase their letter rate by up to 60 % or decrease it by up to 20 %. Ceylon was one of the countries where the foreign letter rate differed from the 25 UPU ctms standard.

The foreign letter rate was 35 c. Assuming that this rate was equal to 25 UPU cmts, double the deficiency (20 c) would have equaled 14.3 UPU cmts. As only 13 UPU ctms were endorsed the exchange rate must have been different, i.e. Ceylon’s foreign letter rate was set below the UPU standard.

Starting, as endorsed, with an exchange rate of 20 c = 13 UPU ctms (or 1 c = 0.65 UPU cmts, a straight forward calculation shows that the correct exchange value of 25 UPU ctms must have been about 38.5 c, which means that Ceylon had set its letter rate 10 % below UPU standard.

Postage due calculation in country of destination

Since 1943, the British postal authorities ignored any postage due markings in UPU cmts on incoming international mail. According to Michael Furfie,

"The British Form P114 included only rates to Britain, so in cases where the
letter rate to Britain was less than the country's UPU rate, postage due
calculation could not have used the relationship

Country's UPU rate = Britain's UPU rate

because offices of exchange using the form would not have known the first of
these.

Although I've never found any official explanations of how it was done, I
believe that, for the most part, the equivalent of the letter rate to
Britain was calculated at PO HQ (which would have had all the information)
as

Country's rate to Britain x Britain's UPU rate / Country's UPU rate

and rounded to the nearest halfpenny. If Ceylon's rates were 25 cents to
Britain, 35 cents UPU, then the equivalent of this 25 cents would have been
25 x 6d / 35 cents = 4.29d, rounded to 4½d. The relationship 25 cents = 4½d
would then have been used to convert double the deficiency into pence: 20
cents x 4½d / 25 cents = 3.6d, rounded to 3½d.

Unfortunately, no copies of Form P114 seem to have survived from between
late 1957, when the 6d rate was introduced, and 1966, when taxe fractions
replaced gold centimes, so we can't be absolutely sure. No equivalence of 25
cents other than 4½d gives 20 cents = 3½d, though."

Credits
Thank you, Allan, for establishing contact to Michael - and most of all thank you Michael for sharing your expertise.