I have a 4-pin sensor with a Natioinal Semiconductor logo and /831 on the body.
The body has the loose appearance of an 8-pin IC that's been cut in half, through the package center line (leaving pins 1-4 or 5-8) and rounded off. Four flat IC-type leads on 0.1inch spacing, single-in-line.
It was used in a variable reluctance tachometer constructed prior to
1979 - if it were a hall sensor, it wouldn't have needed an air-core coil. I expect that it's output would have been open collector, as the sensor had a three-wire interface with an external pull-up requirement on the output.
Anybody seen data on this type of product from early Nat Semi literature? It could just be a comparator.
RL