I received mine from Promag today only to find out that the retaining pin (part 106J if it were Benelli) was missing from my build. Simple oversight I suppose from a custom product. I talked to Pro Mag and they were willing ot mail me a new stock, but I urged them just to mail the pin so we didn't have to hassle with an exchange.
In the meantime I'm using a large nail that I hacked down to size. The diameter of the nail isn't quite right for smooth transitions from each position, but it is sufficient to hold the stock in place.