Aircraft Instruments Catalog, DC Selsyn Remote Indicating, 1941
Here is an Aircraft Instruments catalog from General Electric dated August 1941. This 34-page catalog was intended for aircraft designers and covers GE's line of DC Selsyn Remote Indicating Instruments. Most major manufacturers of aircraft instruments introduced a line of 'remote indicating' instruments in the early 1940's, and Selsyn was GE's brand.
A remote indicating instrument normally consisted of at least two components: a transmitter and an indicator. The transmitter would typically be placed 'remotely' from the panel-mounted indicator and as close to the subject of its measurement as possible. The transmitters were specific to the measurement desired: position (of flaps, landing gear, etc.), RPMs, pressure, and temperature. The transmitter communicated an electrical signal to the indicator to display the measured parameter.
These were breakthrough instruments at the time, in that they permitted multiple readings (from 2, 3, or 4 transmitters) on one 3.25 inch indicator, thus reducing the space required on the instrument panel. They were also (supposedly) lighter in weight and lower in power consumption than other mechanical and electrical instruments.
The 8.5 inch by 11 inch catalog is in good condition given its age. It includes specifications for each instrument including 'blue prints' for dimensional information.