1 April 1879 to ca April 1888

US foreign surface letter rate 5 c
US Conversion rate 1 US cent = 5 goldcentimes
Swedish conversion rate 1 goldcentime = 0.72 öre for letters, 1 goldcentime=1 öre for postcards and printed matters
US marking shows the amount missing in goldcentimes.
In Sweden the indicated amount was doubled and multiplied by 0.72 for letters and 1 for postcards and printed matters. The UPU intention was probably that 0.80 should be used for letters but Sweden normally used the commercial rate 0.72 until about 1888. The amount due was paid by affixing postage due stamps.

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Unfranked letter

Letter with 3 cent

Letter with 4 cent

Double rate letter with 5 cent

Double rate letter with 6 cent