I would like to bring this up because I have not seen it posted anywhere.

I recently purchased both the MCX and MPX.  I wanted to have an AR-15 stock attached to both.  I purchased a picatinny buffer tube adapter from both KAK Industry and Thordsen Custom.  Both companies state it is for the MCX and MPX.  However, I found when I installed either of them on the MCX, the adapters sit too low to get a proper sight picture with BUIS.  Personally, I think the adapters were designed with the MPX in mind, not the MCX.

I did not want to spend $200+ for the KIT-PSB-X-FOLD-BLK, KIT-PSB-X-FOLD-BLK, or STOCK-X-FOLD-M4-BLK.  I contacted Sig Sauer Customer Service and they offer their own buffer tube adapter, ADAPTER-X-FOLD (UPC 798681551163) for $100.  I purchased 2 of them with 20% discount code and free shipping for a total of $160.  They work perfectly.  I was able to install a mil spec buffer tube without any issues.  The only con with them are, unlike the adapters from KAK and Thordsen Custom, there is no alignment holes on the bottom of the adapter to have proper indexing of the buffer tube.  Therefore, you will have to eyeball it to make sure the AR-15 stock is straight and perfect.  I do not have a picture of the adapter right now because I am at work, but just look at the STOCK-X-FOLD-M4-BLK and picture it without the buffer tube, castle nut, and stock.

pics of these please.

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What's with the zip ties? I thought Sig was going to release a hub for a standard AR lower to accept the folder they offer? I want this not for an AR but for a MDT LSS chassis and my Tikka.-want a folder that will stay put while folded.

California.  I am still looking for a better solution.  I do not want to keep the folder open permanently.

Maybe find a thumb nut to replace that screw and treat the stock as a "take down" Slide it on and tighten the thumb nut? What would be cool if someone could figure out how to add a key lock as on a PTO shaft to the stock hub, would be great for you there and if not wanting or able to have a folder.

An example of the lock
http://thumbs2.ebaystatic.com/d/l225/m/mGNkJuyN0657q-Enw9s5SYw.jpg

Bullet Button offers a kit to make the folding stock open.  It a thumb screw with two washers and a nut.  However, on the bottom of the folding mechanism on the adapter, there is a c clamp ring.  The ring is encased and I do not know how to remove it.