Those are "postage meter" stamps. You might Google "postage meter stamps" just to get an idea - they are the same as what you can find on US mail from big companies https://en.wikipedia.org/wiki/Meter_stamp
Actually, these are computer vended stamps also known as ATM stamps, sold from machines run by the Papua New Guinea post office. These are known as Frama types from the company that made the machines. This design was first issued in 1991. Whether these machines are still in use is unclear.
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