From an article by Massad Ayoob for the magazine American Handgunner:
The EDC magazine bodies are from the Walther PPQ pistol, with Wilson Combat butt plates. Interestingly, my own PPQ magazines would not lock into the Wilson pistol, even when I swapped floorplates. The cut for the mag release button apparently has to be slightly different. Bill Wilson told me, “I picked the PPQ mag because, number one, it’s proven reliable, and number two, it’s the shortest 15-round 9mm magazine available. Starting out designing a new gun around a proven mag is a lot easier than trying to design a new gun and a new mag at the same time.”