20c aerogramme returned for additional postage, but posted underpaid a second time and sent to Pakistan on 17 March 1976. The aerogramme was initially returned to the sender for paying the missing 5c to the 25c aerogramme rate for Pakistan. Endorsed „Returned for 5c … postage“. However, the sender did not pay the additional postage and posted the underpaid aerogramme a second time.
As the aerogramme was only prepaid at 20c, it was taxed „10/15“ (double deficiency as the numerator, international surface mail rate as the denominator according to the Vienna system) when posted the second time. This is incorrect as the Lausanne system, which required the single deficiency in the numerator, was in place since 1 January 1976.
In Pakistan, based on the „10/15“ tax fraction a postage due of Rs 17 was calculated.
