/added 10 Jun 2006
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.

Letter with 3 cent. Correct rate 5 cent, Shortpaid 2 cent. US due handstamp indicates amount missing 2x5=10 centimes. Doubled and converted to öre at the rate 0.72; 2x10x0.72=14. 14 öre in postage dues affixed on back. The fact that the postage dues are uncancelled may indicate that the item has not been called for.