It's not that magic. It's the address of the (RF disc) IPL in the "UNIX ROM". It allows the operator to bootstrap Warm UNIX into single-user mode by just pressing LOAD then START without adjusting the front panel switches. Bootstrapping Warm UNIX into multi-user mode is achieved by just toggling one front panel switch (#15) in between LOAD and START. It's all described in the accompanying boot(7) manual page.