You might also like to know of an undocumented instruction specific to the 360/20. This was a form of the Diagnose (opcode X'83') which allowed me to read the hex dials on the console into storage.
This was extremely useful as I had the job of converting 1401 Autocoder into 360/20 Assembler and this instruction enabled me to replace the sense switches. On later machines under DOS I used the UPSI card. Forgive me, but it's 50 years since I coded one of those and I can't remember the operand, but I'm sure someone will.
