The Vaco Products Company is no more; it was acquired by Klein Tools in 1986.
This tester would be for probing for the presence of B+ voltage in automotive troubleshooting work. You'd connect the alligator clip to chassis ground. Anytime the probe tip contacted a B+ energized point, a light inside the handle would go on. Conversely, you could connect the alligator clip to B+, and probe for the presence of a ground point. Here's a view of the tool's innards.
The light bulb is a No. 1816, 13.0 volt miniature bayonet type. On a six volt vehicle, the light would be dim, but still useable. (Six volt vehicles are a distant memory now. I recall a 1949 Mercury pickup truck, and a Ford 8N tractor that had six volt systems.)
This can go in a bin where I keep some miscellaneous electrical gear.
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