I know your frustration, 68's seem to get the least attention. As stated, my car had the lower valence riveted. If you look at the aim 11-13 A6 revisions rivet was removed 11-1-67 (item 5 blank) so that says to me ALL grilles were riveted to that point and just proves (not that I felt it needed any proof) the FACT that mine was riveted. My car is a 12D so some could say it should not be riveted but it also took time for those revisions to go into affect, especially in the LA plant since they were not a GM managed plant and they might have just been using up stock or waiting for the different hardware to arrive. A lot of people do not pay attention to the revisions and assume these cars were all built one way from start of production to finish or everything was done ONE WAY. You have to remember these were MASS PRODUCED CARS!!! I have found a couple of discrepancies with the AIM and the way my car was, and know others have too. Some were confirmed with the revisions though when I started paying attention to that. I wouldn't go too crazy over the hardware issues or let them keep you from attending the show. I have gone NUTS (no pun intended!) to put my car back exactly how it was paying attention to minor, minor details and most of them are never even noticed by the judges-but I don't care because I'm not trying to impress anyone other than myself. Even if you put everything back exactly how it was I guarantee someone will tell you something was not like that or something is wrong. Don't sweat it, you just have to decide where you want to go with the car, what level you want to take it to, and who's opinion of "correct" you want to follow.
Anyway, sorry for the rant, just passing on my experience. Here’s the photo of the bolt for the bumper bumper bracket. It's an M in a hexagon with 3 lines, 5/16-18. Couldn't get a shot of the other side but it's a pointy bolt and just a lock washer and regular nut all black phosphate.
Hope it helps & hope you get to make the show!