The CPU was called MBB (Microprogrammable Building Block).

Here's the C/30 Programmer's Reference for the "Native Mode Firmware System". That was the software that ran in the native 20-bit mode rather than emulating the 16-bit Honeywell mini.

https://walden-family.com/impcode/c30-nmfs-programmers-refer...

I have working PDP-11/23 in my possession and last couple of months I’m trying to convince myself that I have enough soldering skills to solder four additional memory lanes on its backplane to increase memory limit from 256KB to 4096KB. Otherwise even though I install a processor able to work with 22bit addresses only 18bit addressing will be available.

IIRC MSP430 microcontrollers do something like that, too. They have low-power 16-bit CPUs, but you can't do a lot with 64k memory space so newer models have instructions with 20-bit addressing.

They're fun to work with; some versions have FRAM instead of Flash memory.

FRAM is pretty neat. I considered using an MSP430FR5969-SP for a component on a satellite but backed off when I learned more about the destructive read process. We were concerned about a transient bitflip during the write back and had no where safe to store program code other than the FRAM itself. Instead, we went with a RISC-V core in an FPGA and used an 8 channel ADC to cover the telemetry reporting the MSP430 was supposed to handle. I spent a lot of time writing fast, safe code for it so was a little disappointed it never made it into the final design.

The Unix machine was called C70, right?