It appears that the model has a vortilon which was only found on the DC-9-10 or -30 series I believe. Just in front of the main gear you can see the vortilon jutting out.

Here’s a picture a DC-9-10 where you can see the vortilon.

http://www.airliners.net/open.file/585966/L/

…and a picture of the AA DC-9-87

http://www.airliners.net/open.file/156245/L/

EDIT: Upon further looking it seems that the vortilon has stuck with the whole DC-9 series and is still visible on -80 series as well albeit it tough to see unless the leading edges are up which is pretty rare to get a picture of.