not sure if that is the correct terminology, and i should probably post a pic, but i'm looking for the side rail bar (whatever) that clamps a riser rail (and some optics) up tight when mounting to the upper receiver of an AR-15.  its the side that is loose, and is bolted up tight attaching the rail to the receiver.  i've looked in brownells and other gunsmith catalogs, and cant seem to find them anywhere....

anybody got any clues besides contacting some of the optics companies or folks that actually make the rail risers?

TIA