UK: Decimalization - Stamps not Valid

This UK 9 d aerogramme was uprated with a 1 p stamp and sent from PRES… in June 1972 to Penang, Malaysia.

Pre-decimal stamps were, with few exceptions, demonetized February 29, 1972. Hence, the 9 d  aerogramme was regarded invalid.

The aerogramme rate in 1972 was 5 p, so the aerogramme was short paid 4 p. The tax fraction endorsement “4 / 5” is not correct, as it should have shown double the deficiency in the numerator. The single deficiency rule only started with the 1974 Lausanne UPU congress regulation that came into effect January 1, 1976. The denominator correctly shows the first weight step international surface letter rate of the UK at the time of posting (5 p).

As less than half of the required postage for air mail transmission was prepaid, the aerogramme was diverted to surface mail as is indicated by the two-line handstamp “Insufficiently Prepaid / For Transmission by Air” and the the parallel bars crossing out the “By Air Mail”  routing instruction.

Upon arrival in Malaysia, the (incorrect) tax fraction “4 / 5” had to be multiplied with Malaysia’s first weight step international surface mail letter rate (30 c). The result, therefore, should have been 24 c. However, only a 10 c postage due stamp was affixed.