by Postcards and picture postcards which were under-franked were presented in a window envelope to the addressee. This because otherwise there was a chance that the contents were read without paying the postage due. These window envelopes with postage due stamps glued on are not very nice items in the collection and besides it is impossible to check what the rate was, except when the contents are still in the envelope. Here two examples, two different envelopes.
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