Viewed at about a 20° angle...

Looking straight down into the engraving...

...and, again at about a 20° to 25° angle...
I guess a type of graver chisel producing such an angle at a certain depth along with both pigments is what we're seeing. Though, I can see that being the case with the horizontal engravings but, I have no idea how the effect is produced in the vertical portions of the engraving.