Before the handheld DMM this ws the general purpose lab instrument used for common electrical measurements. It can be powered from an AC line using the standard IEC power cord (110 or 220) or from an internal 22.5 Volt Battery. There is a carrying handle on the top so it can be used in the field like the DMM is used now. Maybe it could be called an AMM "Analog Multi Meter". This is a solid state meter that measures AC, DC+, DC- Volts and Ohms. It has a mirrored analog meter. It's 1/3 Rack wide & will mount in 4 Units of height. The black common 5-way binding post is isolated from the instrument chassis. The AC measurements are average responding not true RMS. I think the reason for the DC+ and DC- functions is becasue this meter reads up to 1,000 Volts on the DC range but may have a lower spec for the isolation between the black common 5-way binding post and earth ground. Some power supplies, like are used for small Cathode Ray Tubes (CRT) have the positive terminal connected to the chassis and the negative terminal is floated. The 427A could measure these voltages either dired The construction is discrete components on double sided printed circuit boards with plated through holes. The PCB on the right side looks as if it should be plugged into a socket (it has the protruding board with the traces to act as contacts) but instead there are individual wires with sockets on each that are plugged into pins. My guess is that (1) the PCB form factor was used on another high volume production instrument and that using the same form factor was a cost savings or (2) testing the board was done using a socket but the actual 427A meter used a higher reliability Gold to Gold contact arrangement for reliability. Battery: The battery compartment is under the top cover (remove three screws to access). The battery is an Eveready 763, RCA VS102 and is rated 22 1/2 Volts. It appears to have two binding posts on top on about 1.7" centers, based on the holes in the battery hold down barcket. Three of the Radio Shack 270-391 battery holders (4 ea. "AA") can be used to make a 16 cell "AA" battery adapter. When the rear panel switch is in the Line position and the FUNCTION switch is in the BATT (15V min) position, the meter reads 23.2 Volts on the 30 Volt Full Scale range. 16 Alkaline "AA" cells will provide 24 Volts and 16 NiCad or NMH will provide 19.2 Volts.