1 Jul 1972 to 1 Mar 1974
US foreign surface letter rate 15 c
Swedish foreign surface letter rate 100 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 100 rounded up to nearest 5 öre was normally paid by affixing stamps on the item.

Postcard with 6 cent. Correct rate 10 cent. Shortpaid 4 cent. US due handstamp indicates double deficiency 8 cent and foreign letter rate 15 cent. 8/15 of 100 öre = 53.33 rounded up to 55 öre have been charged in Sweden. The amount should have been affixed on the card but the item may have been refused, the due ignored  or the amount may have been affixed elswhere.