[gumby]

I think most people would not understand the technical term “cover” while “stamp” clues them in that is is likely somehow postal related.

[stordoff]

"postal covers" (the article title) would seem to handle that.

[mgkimsal]

in my 40+ years of living in english speaking countries and sending thousand of pieces of postal mail over that time, i've never heard the phrase 'postal cover'. 'postal' is the only clue that indicates it's ... to do with 'postal' stuff.