/added 3 Jan 2009
Postage due items from USA to Sweden
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.

Unfranked letter. Correct rate 5 cent, Shortpaid 5 cent. US due handstamp TNY. On unfranked items an amount was often not indicated in USA and the Swedish post office knew that 40 öre was the amount due in Sweden.