It's called a 'wriggle cut' in engravers terms. Seen a lot in jewelry and Western art type engraving.
Parker VH, Browning A5 borders, AH Fox borders, are a couple common places it was used on firearms in production. There are others of course.
It's a way to even-up a relief cut background and add a matted surface to it by going in a couple different directions over it.
Generally done with a common flat graver. But occasionaly you will see gold or silver inlayed lines gone over and wriggle cut to enhance or 'bright cut' them. That's usually done with a round nose graver thats the width of the inlayed line.
The so called 'Pawn Shop' engraving on handguns that was popular from the Southwest USA in the early 1900's was done primarily all with wriggle cut to form not only borders, but also flowers, scrolls, figures and other designs.
Wolf & Klar (Pawn, Jewelry, Guns, you name it,,)of Fort Worth Tx was a well known origin for that type of engraving on handguns and their name is often given to the style.
