What metal is under the stainless layer when it wears of?
1 =no , 2= no, 3 Likely brass , 4= no. The buckle is likely a brass although the bottom edge where you polished it is copper color . That could result if you used some cleaning agent. It could be made from copper but modern mass production would use brass. It was plated with nickle and / or chromium. Nickel plating is usually put down first , under chromium plating.