[awh]

I suspect some of the old, bearded and toothless, gnarled and nasty, grim faced and worn members will have some familiarity with this tool and be able to fill us in on it.
As the acknowledged class slow boy.....I haven't a clue.

Okay, I fit about 50% of that description!

What you have is most likely a spirit level. It gets it name from the fluid used in the vials - mineral spirits, hence the term "spirit level".

If you look closely the vials are probably slightly curved, the main difference here is that the curvature determines the sensitivity and therefore eliminates the distortion that is associated with a straight (one that can be read 360 degree around) vial.

The curved vial generally provides a greater "accuracy" (not sensitivity) than the straight vial. The curved vial must be hand set, making sure that the crown of the vial is up, and must contain two vials in each location so that both working edges of the level can be used.

The straight vial is generally used today because it can theoretically be inserted in any direction around its circumference and should be accurate. This speeds the manufacturing process greatly, thus lending itself to the "economy" style of level so prevalent in all the “box” hardware stores.

 

[awh]

Oops! Forgot to add that the small arrows you see inside the window point to the face that particular vial should be aligned with.