GarageBand was released on January 6, 2004. GarageBand has always included an extensive collection of sounds (loops), software instruments, and amplifier effects. Later in 2004, Apple released this first add-on pack to GarageBand, Jam Pack.
This is the original retail box for the first GarageBand JamPack from 2004.
The box indicates that this Jam Pack adds more loops (over 2,000), additional realistic instruments (over 100), custom effects (over 100), and 15 new guitar amplifiers.