Most likely you had a 105a alternator... However, as stated, there was a 120a option. Two ways to find out:
- check the embossing/stamp on the alternator - it'll be on the firewall side, IIRC.
- give the VIN to your dealer for look-up
My personal recommendation would be to upgrade to a ** alternator...preferably the large case variety. Factory alt supplies just enough juice for the factory options. Most of us use our trucks to do more than drive down the street...hence more juice on demand is a good thing.