In this DEC PDP-11 Strategy Memo from Sept 1974, there is this curious question.
HOW MUCH OF THE DEC SYSTEM 10 SOFTWARE
CAN THE 11/85 USE, OR DOES THE 11/85 HAVE
TO START FROM SCRATCH?
(All caps in original)
What was the PDP-11/85 intended to be?
Why might it be able to run DECsystem-10 software?
To put this in context, the VAX strategy was still emerging at the time; there was still thought that a VAX really would be an addressability extension of the PDP-11. The same memo talks about extended (i.e., 32-bit) addressing in the 11/44, which did not happen.
Even if the 11/85 was an early designation for what became the VAX-11/780, why might it be able to use DECsystem-10 software? Were they talking about BLISS source compatibility? Or something more fundaemental?
Or, to summarize: what was a PDP-11/85 expected to be?
Edited to add: so far I've failed to find any mention other than the one that sparked this question.