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I have one that has been stamped without ink and one looks like U91 reversed inked on inside of the envelope with reversed (After … days return to). I need some more info on them. Thank you Richard
Envelopes with colorless indicia from sheet fed presses are called albinos, and typically are valued in the single digit range.
If it is from a web fed press, then it is called a missing color (or color omitted) error and typically are valued in the three digit range.
The reversed image on the inside is called "offset". You get those when an envelope blank fails to move through the press and the ink is instead smeared on the platen. The next guy though then gets the offset image on his backside.
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