From The Science and Engineering of Materials by Donald R. Askeland
Aluminum alloys can be divided into two major groups: wrought and casting alloys. These groups can be divided into two sub-groups: heat-treatable and nonheat-treatable alloys.
The first number of the designation specifies the principle alloying elements. The remaining numbers refer to the specific composition. The temper designation (T or H) refers to heat-Treated or strain-Hardened. Other designations include annealed (O), solution-treated (W), or as-fabricated (F). The numbers following the T or H refer to the type of heat treatment or amount of strain hardening.
Wrought Al Alloys: 1xxx Commercially pure (>99% Al) 2xxx Al-Cu and Al-Cu-Li 3xxx Al-Mn 4xxx Al-Si and Al-Mg-Si 5xxx Al-Mg 6xxx Al-Mg-Si 7xxx Al-Mg-Zn 8xxx Al-Li, Sn, Zr, or B
Casting Al Alloys: 1xx Commercially pure Al 2xx Al-Cu 3xx Al-Si-Cu or Al-Mg-Si 4xx Al-Si 5xx Al-Mg 7xx Al-Mg-Zn 8xx Al-Sn
I do not have a good reference at this time that relates the last digits with any specific composition. If anyone does, please notify me so I can add it to this FAQ.