From Wiki

General Electric GEN-15B

• Type: Straight Custom
• Size: 11x4 Inches
• Material: Plastic Vinyl
• Scales:
  • Custom, C and D scales included

The "Black Body Radiation Calculator", produced by GE's Light Military Electronic Equipment Department and advertising their "Infrared Detection for Searching and Tracking Airborne Targets". Custom scales relate temperature with incident energy, emissitivity, photon flux and several other factors, in english and metric units. A small C and D scale is provided in case you left your real slide rule at home. Includes relevant equations printed on the unit, an example card and a heavy paper slip cover. From a production run of 2500 made in September 1956.