Fastener Needing Identification

[COPOZ/28]

I found this one, loose, hex head screw wedged between the upper rounded edge of the washer reservoir and the  radiator closure panel on my '69 Z28.  It's 1/4"-20, non-full thread, 7/16" hex head, 13/16" long with a small hole drilled through each flat on the hex and a larger dia. blind hole, about 1/8" deep, in the center of the head.  Anyone recognize it as a legit Camaro fastener?

At first blush, it resembles the two fasteners that attach the left side radiator support to the radiator closure panel, into well-nuts, but the those screws have a flange head and do not have their hex flats thru-drilled, per the AMK website -- unless I'm wrong about that.

Thanks for the help.

[bcmiller]

I don't think that's an original assembly line fastener.