Only 2 wires on Inductive sensor?

Hello! I am planning on tearing down my Folgertech Prusa i3, and building a Smartrapcore. In the BOM on thingiverse, it listed a LJ18A3-8-Z/BX for the inductive sensor. I found a listing on eBay for this sensor, and purchased it. From every guide I have seen, there should be a brown, a blue, and a black wire. (+, -, and Sensor) Oddly enough, when I received my sensor, it only had two wires, the brown and blue. I am very confused about this, as I have not been able to find any other information about inductive sensors with only 2 wires. How should I wire the sensor? I have pictures of the sensors below:





Thank you!

At a guess the GND is the metal shell of the sensor (the missing black wire)

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Dust
At a guess the GND is the metal shell of the sensor (the missing black wire)

That would make sense. I will report back after testing.