What are the threaded holes in the rails for?  Is that where the curb feelers go?

According to KAC they are for attaching some form of camera device to the rail if the need should arise. They are threaded in the same universal pitch that all cameras (for tripods and such) are.

K

Yes, the 1/4"-20 threaded holes will accept the screw usually part of a camera tripod head, but then that only applies to the lower rail.
The complete answer then is universal adaptability.
You must remember that before the users had these rail systems, they were not sure exactly what they wanted or where they wanted it (them).  So we tried to design in as many "growth potential" features as possible.
For example, the 1/4-20 holes on the top rail allow for the M203 Leaf Sight to be secured in a real simple manner.  And we make a very low profile PEQ-2 mount (that lets you see more of the front sight post) that uses two of these holes on the top rail (or either of the side rails as well).
Side rail holes may someday allow mounting or securing an improved Quadrant Sight.
We also make a very low profile sling swivel/bipod stud that screws into these holes (and for the FF RAS's, a nut that slides down the opposing inside track for the same purpose as the FF's don't have the threaded holes.

Thanks, guys!

[b]coldblue[/b]...what is the part number for that low profile sling stud?

For the :
Bipod Stud & Nut Assy for RAS Forend:
P/N: 20288
If you use this on a M4 or M5 RAS, you will not need to use the nut that comes with it.  The nut is only used on the FF's, MRE, etc.

Although not new nor enhanced I jury rigged a quadrant onto the side of my M4RAS.  I also have used a camera tripod with it (the M4)

Proving as Dave has stated, a very versatile system.

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=7920[/img]