This is a fitting designed to be inserted into a 1 3/8" hole drilled into the top of an auto top-off reservoir or other covered container fitted with a Neptune System Water Level Sensor.  It allows the sensor to be mounted in places where the included mounting clip will not work, and removes the need to try to cut an elliptical shaped opening to fit the sensor.

For drilling acrylic or other similarly brittle materials, I would suggest using a 1 3/8" step drill bit such as this one on Amazon: https://www.amazon.com/CO-Z-Titanium-Drilling-Cutting-Electrician/dp/B076QC5BWR/ref=sr_1_3?crid=2WTFFCERE9Z58&keywords=step+drill+bit&qid=1674159767&sprefix=step+drill%2Caps%2C123&sr=8-3 (Note: I am not affiliated with Amazon or the company that makes/sells this bit.  It's just what I happened to use).