I have not encountered such a form of postal cancel so far. The stamp also features the popular blue cancel with the letter N in a circle. But the triangular one is a mystery to me.
The triangular markings I have seen on US postal history (or covers coming here)have been Dead Letter Office markings, but there might be others as well.
The blue cancel is the "string of pearls" precancel of New York City. Tracing from the Oct 1975 U.S. Specialist:
Black triangle marking from the dead letter office, the word "LETTER" is readable on the stamp in original post. Typical example shown from Kelleher sale 721:
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Posted 10/28/2020 11:14 am
