[Much useful info deleted -- Thanks a lot for this]
letter code displayed next to the desired OS. See
<http://www.classiccmp.org/hp/hp-boot.htm> for more information about this.
I've taken a quick look, and I think I have a couple of corrections. You
mention that options are listed by their HPIB address, don't you mean DIO
bus select code? A number of the options aren't HPIB devices.
And you mention the serial number being stored in the boot ROM. AFAIK,
it's stored in a separate 'ID PROM' -- the 9826/9836 boardswapper manual
tells you to remove this chip and put it into the new CPU board if you're
replacing the CPU board. In these machines, only the latest (of 3
possible) CPU/RAM boards and the CPU/MMU board have this PROM.
-tony