It is of course from Colorado Gun Sales.
You can also get a a Sig licensed air soft replica
for considerably less. Here
is a calguns.net review of the replica with pics of it installed.
Basically, both are the same. On the collapsing, folding stock of the 556 you will have to do a little dremel work at the front of either cheek rest for a proper fit and remove it when you fold the stock to get proper clearance. My understanding is that since this rest was made for the Swiss non-collapsing stock
, if you put one of these stocks on your rifle, then you will have no problems with a drop in fit. However, the main problem is that the Swiss 55X rifles have AK-type mag releases (forward of the trigger guard) and either stock on a US lower with that raised button M16-style mag release (on the side of the receiver) will not allow the stock to lock in place when folded with the cheek rest on. I have yet to find a cheek rest made specifically for the US collapsible folding stock.Here's
an older thread covering the topic. You could probably just PM the guy who did this to see how his cheek riser is holding up.