Ah ha, I found it!
I knew something was askew while I was looking through last years rule book. For a long time there was no info regarding splitters in the rules. Well, in 2009 that changed. Prior to 2009, the rules used to say that an air dam was supposed to extend STRAIGHT down. I think this is the main reason why all the photos I posted are of air dams that do in fact go straight down. Well in 2009 the part about an air dam extending straight down was left out for whatever reason. I don't think the rule changed and I think they are in fact supposed to still extend straight down but it obviously doesn't say that any more.
Am I wrong in saying that the air dam in question extends downward and then rolls under?
Is there a bumper bar behind all that white and chrome business on a stock truck? If so, then according to the SCTA definition, which says that a bumper is metal, all that that is covering the bumper bar is a fascia and considered to be part of the body. I don't think that little strip of chrome is actually the bumper. The whole white and chrome thing combined is either the bumper itself or it's a bumper cover. I couldn't find any pics of a Duramax with the front bumper removed to verify and I don't know anyone that'll let me take off their bumper to check.
Have any pics of a bumper-less Duramax?