Re: EDM Windrunner rail stiffener
I haven't seen that one - probably a custom job. Bill's been making available two products along those lines for several years now. One is a U-Shaped piece that lays over one of the stock's rails and physically prevents it from collapsing (if you get in the habit of not fully locking the stock). The other is a stiffener that connects the two rails, similar to the picture above, however much less engineered and simple.
Personally, I never had a need for either.