From Wiki

Concise / Sama & Etani Metric Marvel Data Center Model 200

• Type: Circular
• Size: 3 Inches / 8 cm
• Material: Plastic
• Scales:
  • Front: D / C CI L A S T K
• HvH: 3192

Circular rule mounted on 3 inch by 4 inch card. Bevelled slide circle. Includes "Insert MM" metric conversion card in a sleeve behind the dial, and a "Reactance Nomograph" on the reverse. Evidently, the rule originally came with the "Insert EE" marked with electrical formulae. A previous owner or dealer substituted the MM insert. Inch and centimeter scales on body. Cursor marked with scale designators. With white vinyl case and manual. Designed by Sama and Etani, Inc., Groton, Massachusetts. Gift of T. Elam.

Notable about this rule is the big blank space at the top. Many of these were manufactured as advertising specialty items, and this was likely on the way to the silk screens one day early in its history.

I remember studying LC circuits in high school and college. Boy, would this have been handy then! Impedance here, frequency there and bang! "Every little picofarad has a nanohenry all its own." -- Don Vonada

The manual (copyright 1983!) says: "Electronic calculator or not, you still need specialized Data Centers from SAMA ASSOCIATES ready for calculator input or for use with our built-in circular slide rule." Shows how the world was changing. The big deal was the conversion table, the slide rule an "add on". Heaven help you if you get caught in the rain without your TI-30!