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impressed stamp
noun
Definition of IMPRESSED STAMP: a stamp (as for postage or revenue) printed directly on a cover, document, or other paper bearing it."
Postal stationery would cover nearly all items such as envelopes, wrappers etc. that are whole.
Impressed stamp covers a stamp printed directly onto postal stationery... embossed stamp would need a separate entry for themselves.
So...you end up with....
Postal stationery impressed stamp cut squares (or cut-outs as some might wish) which is almost "cut squares of stamps printed on envelopes" as you had at the start.
