I recently found this unusual tester at a yard sale.

Oddly, It's not one that I remember seeing there back when I used to be a frequent customer. As far as I can tell, it only appeared in the Radio Shack catalogue in 1989 and 1990

Let's see how it works and what's inside.

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This isn't the only way to do it. It's probably not the best way to do it. But it's how I did it.