I wonder if,
maybe, it used the same protocol as the GT40, which also
had a boot-over-serial line capability.
Section 5.1.1 Bootstrap Loader describes the packed-and-serialized
6-bit "byte" stream
I have the GT40 boot ROM assembled on a PDP-10 host and used for booting
by SIMH. There is also a GTLOAD program which translates a PDP-11
binary on the PDP-10 host and sends it to the GT40.