Canada
can-20150000-b-002
page = 1
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Width: 238 mm
Height: 104 mm
Location: Box 1002
Stamp ID = 6974
The Charlottetown Islanders are a Major Junior hockey team based in Charlottetown, Prince Edward Island playing in the Quebec Major Junior Hockey League from 2013 to 2022. The team played in the Eastlink Centre. The team secretary must have been traveling on the team bus and used a US stamp to mail this while out of Canada.
From Linn's "Stamp Issuing Entities Of The World" page
Canada (1851-)
Stamp-issuing status: active; Population: 29,123,194. An independent state within the British Commonwealth, occupying the northern part of North America.