OMG. You are the only one I’ve encountered who knows that besides myself. They could use the 370 to run 1400 code without emulation. Then they could revert it to the 370 ISA. How cool was that?
My fav was the Boroughs B1700 which had a different set of Microcode depending on what language you were running. (Cobol/Fortran etc)
This is back in the day when computers actually were fast & worked and you could navigate to the moon in 8K of RAM.