cellbio,
The DSO is located on the door tag, look at the driver door just below the latch there should be a metal plate with numbers on it. The top number should be the cars VIN number. Then below that you will see 7 different areas: Body, color, trim, date, DSO, axle and trans. Look at the 5th set of numbers and it should be one of the DSO (domestic special orders) numbers that sold GT/CS's...they were 71, 72, 73,74,75, 65, 61, 62, 53, B6, B7 and 51 ( a 51 would actually make it a HCS). Here is a link to plug in your cars numbers and find out info on it:
http://www.fomoco.com/mustang-vin-decoder.shtml
And since these cars are becoming so popular, there are becoming many that are being faked. There is only one (well maybe 2 if you have all the original paperwork) way to tell what a early mustang came with. Kevin Marti has the "rights" to Ford info which he then reproduces your cars info, here is the link from this site on it (scroll down about half way):
http://www.californiaspecial.com/resources.htm#g
The report should state the "options" and will say "California Special"....it is well worth the effort to prove your car is a real GT/CS.
Who knows may be the report will tell you that the car is some type of special "Shelby" car. I don't think it is just a very good looking California Special.
I imagine the reason someone state the car was a Shelby Cal. Special was because that most of the fiberglass on the Cal Spec came from the 68 Shelby Convertible (side scoops on GT/CS are not cutout). The car has been known as a "poor mans Shelby" but none of us that own one think that....instead a very rare and unique 68 Mustang.
Good Luck with your (or should I say your wife's?) car.
Doug
PS.. Keep the question coming, there is always someone here that can answer them and don't feel bad about not knowing much about the car, everyone here was in your shoes at one point. The reason Jon started this site was to inform people about the car....you have come to the right place.