Neither Brownells or Midway had the factory rear M&P sight in stock when I checked the other day, so I decided to call S&W. The CS person told me over the phone in the most serious way that the rear sight is a restricted part that only an armorer can buy. When I mean in a serious way, you would of thought I asked him if they could sell the blueprints to up-coming projects.

Luckily I checked Brownells again last night and saw they had it in stock again which seems anyone can buy.

REAR SIGHT | Brownells