Yes, but it was built in December of 1978. You have a 1979 MODEL YEAR. I have a 1984
Hurst/Olds built in November 1983. It's still a 1984. You said it was built in December. If it's a 1979 model, it HAD to be December of 1978. It's the only way possible to fit in that model year. Do you still have the door jamb decals? It would have 12/78 as the build date, guaranteed. Sounds like the body tag had a 12C on it for the 3rd week of December----1978. There's no way in hell your car was built in December of 1979 and still be a 1979 model.
The only way your car could be built in December of 1979 and still be a 1979 model is if GM extended the 79 model year for some reason. But since they made 1980 LeMans cars, I'm betting they didn't. If it were built in December of 1979, it would make it a 1980 model.
For GM, model years NORMALLY would run fall to fall (August through July is the typical model year) with some exceptions. On occasion they would pull some model years forward and extend some past the fall. I know Camaros did that a few times. Built 1969s until November because the 1970 wasn't ready, and then for 2002, they brought the start date of production for Camaro/Firebird up to April of 2001 and the last ones rolled off August 2002. And then when they brought back the Camaro for 2010, they started building them in February 2009. I ordered mine in April 2009 but demand was so great it didn't get built until October 2009. But it's still a 2010 model year.