Originally Posted by piston slapper
???? I dont know for sure....????
I've had it for years and never used it...
Think I got it with some automotive tune up tools I bought...
Long time ago...
Unless someone else is wiser...
You Got It.
Here is the link to repair parts for Sun Electric tools and they show the exact tool you have. They only offer the replacement lens.
Here is a link explaining how it works, although the gauge in this link is from another mfg