Back to the thumb pad of the example top lever. I believe I'm looking at metal checkering rather than purely ornamental engraving. It was probably cut with engraving tools though, and the uneven depth and spacing, to me, give it a reasonable sign of hand work rather than some other production machine.
I think, to some extent regardless of the technical quality, the original maker felt the entire gun was worth time and expense to tend to details. I believe the original thought was it could be a quick point to evaluate to help decide if a gun overall was worth a closer look.
