🔍 What It Is:

Those small glass tubes containing three tiny ball bearings are not random curiosities—they’re precision components from an old spherical (bullseye) spirit level, commonly used in: Carpentry
Masonry
Machinery alignment
Surveying equipment

Unlike standard linear spirit levels (with one curved tube and a single air bubble), bullseye levels use a circular, dome-shaped glass capsule filled with liquid and often multiple small steel balls or a single bubble to indicate levelness in two dimensions at once.
However, in some older or specialized designs—especially European or industrial models—the “bubble” is replaced or accompanied by tiny steel ball bearings that roll to the lowest point, helping the user visually confirm true level from multiple angles. In high-vibration environments (e.g., machinery), bubbles can be hard to read.
Ball bearings provide a clear, physical indicator of gravity’s pull.
Some antique levels used three balls to triangulate levelness more precisely. 🛠️ Where You Might Find These:

Inside an old wooden or metal leveling tool (often brass or cast iron)
In a machinist’s toolbox
At estate sales, flea markets, or inherited workshop drawers
If the glass tube is dome-shaped, sealed, and mounted in a metal ring, it was almost certainly part of a leveling instrument.