Quick Google search came up with this:
Help! Need retaining screw size for Magpul PRS stock **RESOLVED - AR15.Com Archive
"The screw you need is somewhat hybrid.
1/4x28 panhead machine screw 9/16" long. The slot head measures 1/2" wide.
Black Oxide/Blued with red locktite.
The hybrid part...There is a 3/32" hole all the way through the center of the screw for lubrication of the receiver extension internals.
Note: Magpul says "DO NOT use any other screw to mount the PRS as it may affect weapon cycling or damage the stock."
(A panhead machine screw head is thinner & wider than a standard bolt head.)"