Im still using corn cob and a light abrasive car wax in a vibrating bowl. But ive noticed that there are magnets sold to sort the pins from brass, and while a high grade of stainless such as 304L is not magnetic, this means the pins being sold are (1) a low grade of stainless steel or (2) that they were able to electro plate a stainless jacket over a base material. And this is common practice these days just look at the flatware Target is trying to sell these days.
The OP did mention they were the cheapest he could find so im suspect of (2). I suggest the original poster take a shot glass of 50 or so pins and a half a shot of your washing solution and let them sit over night and see if you get darkened water in the morning.