​A problem was that it was 40 class processor and as you says that means it was shared I/D (i.e. pure 16 bits) - so it lacked the 45 class 17th bit.   The 60 has went into history as the machine that went from product to "traditional products" faster than any other DEC product (IIRC 9 months).   I'm always surprised to hear of folks that had them because so few were actually made.  

I've forgotten the details nows, but they also had some issues when running UNIX.  Steve Glaser and I chased those for a long time.  The 60 had the HCM instruction sequences (halt a confuse microcode) which were some what random although UNIX seemed to hit them.  DEC envisioned it as a commercial machine and added decimal arithmetic to it for RSTS Cobol.​  I'm not sure RSX was even supported on it.

Clem