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This is a replica of the 90's Bally / WMS Spirit Level Bracket that holds the spirit bubble vial to aid the owner / operator to set the level of the playfield correctly.
Vials are still available (e.g. Marco) but the original bracket seems to be very limited and expensive at about $15US each with a vial. This is now an alternative if you want to have something in your machine if the original is broken or missing.
Or you can purchase a different colour other than black to match you pinball colour scheme e.g. Blue to suit a White Water etc.