Griswolds are fantastic - I heard that they were made using a different process to the modern sand-casting and the surface is much smoother.

I believe the difference is that the inner surface has been milled flat using a milling machine.

One thing I wonder is if it's possible to get similar results by buying a cheap Lodge skillet and flattening the surface with a milling machine or a random-orbit sander.

I have about six pieces of cast iron. I milled one of them flat. I didn't do the others because the performance increase wasn't enough to be worth it. But my diet is full of fat and oil so ymmv.

It does appear that it was a post-casting milling step (the inside is smoother than the outside and handle), so you probably could replicate it. However, cast iron dust is abrasive (so it’s a pain for machine operators) and legacy cast iron is still fairly plentiful so it’s probably not worth the time and expense unless you were doing it for curiosity rather than to end up with a finished pan.

Agreed - I have a 10" Griswold griddle that's got milling circle marks on the smooth surface.