The Tektronix M120B (P/N 155-0109-00,155-0109-01) is a trigger monolithic integrated circuit (good to 350 MHz) in a 16-pin DIP. It was designed by Gary Vance.
It is designed to be used in conjunction with the 155-0126-00 (M121) trigger amplifier and channel switch.
There were two versions, the 155-0109-00 and 155-0109-01. (Differences?)