The idea behind this one is about as basic and as demanding as machining gets: take a big mold or tooling die assembly and work the steel face until it behaves like a mirror. The finished surface reads as optical-grade, and it clearly picks up a person and the surrounding workshop in its reflection. That is a serious result on a part of this size, not a small polished coupon on the bench.
What stands out to me is how much is going on around that polished face. The surface has curved contours, bolted inserts and guide components to work around, which makes a uniform finish a lot harder than polishing a flat plate. The nearby tooling carries engraved markings such as W.NR. 25717, 2510 HRC52-54, 30704, 1410 HRC42-46 and H13 HRC42-46, spelling out steel grades and Rockwell hardness specs. The polished part itself is P20 HH steel.
The method, as the maker confirms, was surface grinding followed by lots of polishing. That is the entire recipe, and honestly it is the only honest one for this kind of result. Getting the grind stage right is what gives the polish a chance.
One open question hangs over the whole thing: whether the customer wants a scotch-brite finish after machining. The maker's reply was a plea of please no, and after this much work on the surface, I completely understand that answer.