I actually see three differences now between my shroud and all of the other shrouds pictured here. Besides the two cut outs that have already been mentioned, one at 12 o'clock and one at 6 o'clock on the engine side, I also noticed the cut out that the top mounting bracket slides under is wider on mine.
I Googled images of '68 Camaro fan shrouds and engine compartments and did not see a single shroud with cutouts like mine. This is a mystery to me. This is the shroud that was on the car when I bought it 1978. It was a mostly an unmolested original car in stock condition. Nothing had been hacked. I doubt anyone was making reproductions at the time when you could just go to the dealership and buy a new one. I can't figure out either what would be accomplished by trimming out those small sections...….Another thing is that the edges of the cutout areas look very "factory." There's no evidence of any cutting like saw marks. The cuts are straight as an arrow too. Anyone who was not familiar with how one of these ought to look would never look at it and think it didn't come from the factory that way.
I am not trying to say that someone didn't trim it. I have to figure they probably did seeing as I can't find another one that looks it. If they did though, they did a great job! Oh, and for what it is worth, I looked again and there is no part number stamped or molded into my shroud.
I guess I'm on the fence now whether to repair this shroud or go after a new GM. I wouldn't try to fill in the cutouts, just repair the cracks. And I knew all along that if repaired it, I was going to have to paint it. So that brings me back to my original question.....What kind of plastic is this? Whether you're buying a plastic repair product or filler material for plastic welding, you have to know.....and maybe no one will ever know.
I appreciate all of input on this and all of the pictures posted.
Dave