1 May 1975 - 31 Dec 1975
US foreign surface letter rate 18 c
Swedish foreign surface letter rate 110 öre
US handstamp shows amount to pay in US cents (normally double deficiency) and US surface letter rate.
In Sweden the indicated fraction multiplied by 110 and rounded up to nearest 5 öre was normally paid by affixing stamps on the item.

Air mail letter with 13 cent. Correct rate 26 cent. Shortpaid 13 cent. US due handstamp indicates double deficiency 26 cent and foreign letter rate 18 cent. Swedish due 26/18 of 110 = 158.89 öre rounded up to 160 öre has been charged. The amount has probably been paid together with other fees.