I have an '89 RS, recently "retired".
The air dam is fairly straight across and
straight up and down. It is angled out
with the bottom closer to the front of the car.
I have an "88 IROC-Z.
The air dam has a definite curve to it top to bottom.
The center of the air dam is closer to the front of the
car than the top or bottom of the air dam.
Like this:
Looking from side of car...
front of car ( back of car.
Which is correct one?

 

Maybe one of them was attached incorrectly - the bolts that attach go on the rear side of the spoiler. Could also be the case that one of them has just warped over time from pressure at highway speeds.

There may be different models as well, but all the ones I've seen were pretty mch the same. I know some cars had a 3 piece setup, that kinda looked like:

front
/-----\
rear

Also, some of the SC cars had the air dam kinda built in, as a part of the nose cone - these cars had no chin spoiler.

There's my limited knowledge - Not sure that I helped you any though