Just so you will know that all the rest of us 68 guys are not ignoring your question, I've looked through two P&As in three different places and come up with nothing. Even if I had, the service part number of the P&A would not necessarily correspond at all. Page 138 in my 68 AIM is clear as a bell and it says "12 9919303 SCREW".
The P&A does not address information in the exact same manner as the AIM, so it is sort of like looking for a needle in a haystack. I would suggest that you contact Ed and ask him. He is a wizard in finding #s in the P&A.
I did find an exploded view of the 67-68 grille that showed the hole in the valence that the screw goes to, but no reference to the screw. That view does mention a few bolts but it does not give a part #. It references their size. The AIM gives a better description of assembly...
If you haven't already seen it, read over Mark's recent, excellent description of the GM parts numbering system:http://www.camaros.org/forum/index.php?topic=5644.0
JohnZ helped me see how this works (or doesn't) some time ago in some of his posts to my questions and comments. I still find the P&A, etc. helpful in figuring out repairs/replacements, but I guess four decades later we do not have a better cross-reference than "grunting it out" exactly like you are doing here in CRG.
Sorry, I couldn't really help....