This is an Analog type Exhaust Gas Temperature Guage for Aircraft. It is used to measure the Exhaust Temerature of the hottest cylinder as a safeguard to engine overheating.
Thermometer with 2 scales. Old-fashioned design. "Fig 11: A well-made commercial thermometer" black and white re-trace from: GENERAL SCIENCE BY BERTHA M. CLARK, PH.D. (Gutenberg EBook 16593)