It is called scraping, or, hand scraping, since it is always done by hand:

https://www.mmsonline.com/articles/the-art-and-science-of-hand-scraping

It is common on hand made French guns from back in the day. Probably the only gun that really needs it is a Darne, with its long slide, but, it is considered a sign of quality and does retain oil. I’ve seen it on some higher grade German and Belgian guns as well. There is a gent, locally, who offers a three day course on how it is done, and he is busy all year long, traveling the country doing it. My machinist friend contemplated it, but, the course is expensive. Too expensive for him, anyway.
Photo of the bottom side of an R10 Darne. Even the ejector hooks have been scraped, which, is purely decorative, but, it no doubt adds precision and smoothness to the ways and bed of the actual slide.

[Linked Image from jpgbox.com]

Congrats. Looks to be a nice gun.

Best,
Ted