I have always woundered where the FM Brass Builders plates ended and the stainless Steel plates began. I had to good fortune to look at a collection this past weekend. In the collect was AC&Y FM plate number16L831. It was brass, I own SP 12L834 which is Stainless steel. Since AC&Y owned 21L832 and SP switcher order began with 12L833, I would think that the Last brass plate was 21L832 and the first Stainless steel plate was 12L833. However , if FM used delivery date as the dividing point you might have a mixing of numbers between brass and stainless steel.