From Wiki

C-Thru PC-81 Profit Computer

• Type: Circular
• Size: 4 Inches / 10 cm
• Material: Plastic
• Scales:
  • Front: Unit Count / Margin, Profit

Obtained in early 2005 from the art supplies section of the University of Washington Bookstore. Intended to be a fast way for merchants to calculate gouge and squeeze, er, I mean margin and markup. Note special marks for dozens and gross. Would work better if the discs lined up.