I believe you are right. I saw a segment on How It's Made and that's how they were making brushed stainless sheet panels. The gunsmith probably used a die grinder with a wire wheel would be my guess.
Bead blast, or like I took the shiny SS finish on my SS loaded Champ down to a brushed finish. I took the parts off until I had flat surfaces, and pushed it one way across medium grit emery cloth like plumbers use.