1999 was first gen, Slot A based. Second generation (Thunderbird) Socket A based Athlons (and Durons) described here were released 2000.
Got one in 2001 and I was so proud of my AMD Athlon Thunderbird 1400 that I even accepted that loud noise from the cooler I used.
Although in hindsight, the money I spend for that luxury would have been better invested in other stuff, improving my life a bit more then a loud and noisy airplane turbine.