What does ‘engine turned rib’ mean on English shotguns?

ENGLISH SHOTGUNS
Mike George
All ‘engine turning’ means is ‘produced by machine’.

Engine turning can produce many forms of pattern, from the coarse lines which might be used on a rib to that pattern of tiny, shiny circles you sometimes find on bright parts ? often on rifle bolts or on the barrel cheeks and lumps of O/Us.

The coarse pattern found on a blacked rib is to prevent sun glare, while the tiny circle pattern is more than cosmetic ? it retains a very small amount of oil.