Now that I look at it closely, I think you misread the top line of the cowl tag. I think it should read:
79 4AG87 Z 088847
It decodes as following:
79 - the model year 1979
4AG87 - Buick Century 2-door sport coupe
Z - the assembly plant (Fremont, CA)
088847 - production sequence #
If you look at the build sheet you will see the sequence #088847 is listed in the upper lefthand box (#1) and the 4AG87 is in the upper righthand box (#9). I believe the Turbo Coupe option was only available on the Century sport coupe (AG87) and not the Century Special coupe (AE87).