The first Apollo/DOMAIN systems were the DN100s, of which 2 were shipped on March 27, 1981 with Aegis SR1 installed on them to Harvard University (information provided by Jim Ward). Documentation for Aegis SR1 consisted of 3 books written by Penny Orwick. The DN100s were discarded by Harvard University a few years ago (Harvard CS Help Desk).
The latest OS that is available for Apollo's is Domain/OS SR10.4.1.2 (SAU7) and SR10.4.1.1 (all other SAUs), consisting of 5 tapes for SR10.4 and 3 tapes for the SR10.4.1 upgrade. Hewlett-Packard offers some patches for Apollo/DOMAIN machines (You are now required to register (free) to retrieve any patches) (Domain/OS SR10.3.5 and SR10.4/SR10.4.1). Those patches bring the OS up to SR10.4.1.2. All the SAUs from SAU1 thru SAU6 (all three-digit machines except the new HP/Apollo 4xx Series) were dropped at SR10.4 and thus can't run Domain/OS SR10.4 operating system. SAU1 (DN100, DN400, DN420, DN600) was dropped at about SR9.7.5 or SR9.7, thus making SAU1 systems (DN100, DN400, DN420, DN600) the only ones that can't run any of the SR10 systems.
The DN10000 machines run a different OS release, with a .p suffix, which stands for PRISM, the name of the DN10000 CPU's architecture (Parallel Reduced Instruction Set Machine), e.g. SR10.4.p. The latest OS for DN10000's is SR10.4.1.2.p, consisting of the 5 basic install tapes for SR10.4.p, one upgrade tape for SR10.4.1.p and HP's patches.