Pressure Altitude Calculator for Rolls Royce Dart 534-2 Engine (NOS)
This is an interesting little gizmo. It is labeled as an "indicator" for use with the Rolls Royce Dart 534-2 Turboprop engine in 1971.
This description is from a knowledgeable visitor, Allen in Canada:
"It is used to calculate the fuel trim setting for the particular temperature and pressure altitude. You align the current temperature with the pressure altitude of the airport. Then read the fuel trim setting to be used for takeoff."
The Dart engine was primarily used in civilian/commercial transports such as the Vickers Viscount in the 1950-70's.
Measures ~ 3 in square.