There was an article 3 or 4 years back in Mopar Action about Frank Badelson's unrestored Challenger T/A, and the rear valence hardware was one of the details they showed a picture of and details on the fasteners.
When I got my car back from the experienced Mopar restoration shop that was skeptical of any input I gave on my project since they already knew everything (sarc), I found they'd mounted the valence with regular phosphate coated bolts, which didn't put a smile on my face.
Frank sells a kit with the proper screws, which look the same as shown in your picture (his part number 70VAL). I replaced the bolts the shop used with the screws and washers from Frank's kit and after touching up the paint marred by the bolt heads, it looks like it should!
My convertible's rear valence is mounted with the same screws and washers.
I assume they used the same hardware to mount valences at both plants, obviously my cars were built at Hamtramck, not LA. Hopefully a LA expert can confirm they are same.
Sorry to go OT, but since things I found done wrong on my restoration is on my mind, I found the shop painted my rear brake backing plates silver or cast iron gray, which doesn't seem right to me. I saw an NOS set not long ago that were black, were backing plates installed at the plant painted black, or are they bare steel?