I've had this happen when a mint stamp was on top of a similar stamp (stacked) During storage the mint stamp suffered some dampness (glue softened) thus some of the image of the stamp below came off with it over time.
you can see your is actually a little off center top to bottom so you can tell it was an ink transfer
it looks like one was on top of the other. you can tell that it was done after the glue was put on, the missing piece shows no imprint, so it would have been done after glue was applied. is that correct?
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