Inside a mechanic's test probe - with schematic

A very simple, but extremely useful 6-24V test probe that can detect polarity and force energise circuitry in either polarity with the press of a button. It's basically a logic probe for cars and trucks.
The circuitry is refreshingly simple, with just one oddity. It turns out that the original this was probably based off does just have the resistors and LEDs as I described. I wonder where the extra components crept in.
This test probe is widely available on eBay. Target price is around 20.
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