This is a bubble level designed to work with M8 T-track systems on truck campers such as Lone Peak, Super Pacific, Tune, Etc.
I tacked the bubble level into the insert with super glue to hold it in place.
10mm X 29mm square bubble levels I used:
https://www.amazon.com/dp/B07VTQG17V?ref=ppx_yo2ov_dt_b_fed_asin_title
The author marked this model as their own original creation.