Well,
NOW, the time travel is going back about
ANOTHER 20 Years!!!!

The ORIGINAL 'Valve-Tester' was just that!! It tested a Valve!!! Back in the early 20's, none of the 'modern'
Octal Base, or 7/9 pin valves/(tubes! who called what/when?) even existed yet! All that existed then, was
the 'typical' '200A' teardrop shaped 4-pin Triodes/Diodes. So a 'Tester' was as simple as this!!!...
'One' would plug the plug into an 'active' Radio, and put the Valve into the unit, to read plate/grid current.
Way before then though, 'typical People' did not have the likes of voltmeters/multimeters, and were using
multiple voltage batteries, and needed to at least know what is Negative or Positive!! Their earlier instructions
were to put both wires into a Saline Solution, and look for BUBBLES on the NEGATIVE wire!!

Later on, advanced technology at the time invented THIS device!!...
where a chemical change due to the polarity, gave an indication of polarity.
Here's another Ad, for another model, that just had a light globe instead of a mA Meter...