Looking for operating information for this tester. Not finding much online.
I cannot tell you if it had any specific purpose but it is an early combination Volt meter and Ammeter. Rather than have a switch to change the range it has instead different places to make connections to it for the various scales. The left side is a volt meter with full scale ranges of .3, 3, and 30 Volts. To read full scale on each range you make a connection to the top left connector with the POSITIVE Connection and then a connection to only 1 of the other 3 namely 3/10 (.3), 3, or 30
Likewise you have 3 ammeter ranges of 3.5, 35, and a huge 350 amps. For those scales the larger +- connection (second one down on the right side is always one of the connections while the scales are either then the top right connection for 350 amps or the ones below that at 35 and 3.5 range. The meter would appear to have 2 separate movements in it that allow you to measure both voltage and current at the same time with appropriate connections. It is for DC only and surprisingly the movements at least "appear" to be not screwed up or bent since they are both only slightly off zero and could likely be placed exactly on zero by the appropriate screwdriver "zero adjust" that each movement has which appear to be labeled "correction". It is possible and even likely that the multiplier resistors inside for each range may not all be still in perfect working order but there is no range switch to break or get corroded. It may have had a specific purpose since one of the ranges appears to measure "cadmium" voltage range which may have been a specific type of battery or electrolysis voltage on perhaps some sort of sacrificial anode. Just my best guess.