antique oak tool box for gauges, machinist's tools or letterpress printer's case

Nice old finger jointed wood box, solid oak, early 1900s vintage, marked Simplex with a worn label on the top of the box.

We've found everything and anything marked Simplex when we tried to research this piece, but our best guess is either a box for machine gauges for industrial equipment or a case for some sort of letter press equipment. It's marked something about moving proportionately to the rate of flow, but the label is not very clear.

The box measures 19" x 5" x 3 1/4" overall. It's well made in in very good condition structurally, though the original shellac finish is worn.