Concise CTCS-552

(Click for a larger image)

Sama & Etani made about a zillion of these plastic pocket-size circular rules under the Concise name, most of them as giveaways for various manufacturers, contractors and suppliers, like the example pictured here. The back of the rule and both sides of a card that slides into the middle of the rule are crammed with reference tables containing unit equivalents, physical constants, chemical symbols, atomic numbers, atomic weights, temperature conversions and so on.

Oh, and it has inch and centimeter scales on the edges, too. It's the Swiss Army Knife of pocket slide rules.