Apple Xserve (Early 2009) - Manual (Page 34)

34 Chapter 2 Installing or Replacing Components
The 12 memory slots are labeled A1 through A6 and B1 through B6, in interleaved
fashion. Single-CPU systems might not have B slots.
(Back)
(Front)
CPU B
B3 B6 B2 B5 B1 B4
A4 A1 A5 A2 A6 A3
CPU A