I have two of the Jwalker (evidently) MPX/MCX pic rail stock/brace adapters I picked up in trade several weeks ago. I was also part of the navalbeaver/Lee Majors group buy for the similar item. I have gone from none to 3 in a short period of time. The two Jwalker ones are up for sale priced as follows...
Adapter alone.... $75 shipped.
Adapter with MPX/PSB brace attached.....$200 shipped.
A picture of the adapter with brace attached is a few posts down when I was selling off a FDE XCR Pistol. Full disclosure on the adapter with brace installed....its shows some bare aluminum underneath where the screws get tightened down from the brace.