This is a typical old-work junction box with integral NM cable clamps. They are each labeled with a tiny "N".
I had assumed this stood for NM and didn't give the labels a second thought. But then I removed this junction box from 1955:
Now the top clamps are all unlabeled and the bottom clamps alternate between "N" and "T". There is no perceptible difference between the "N" and "T" clamps except, when this particular box was installed, only the knockouts for the "N" clamps were used.
What do "N" and "T" stand for?