1M1
The 1M1 is a letter series plug-in used for calibrating a 500-series mainframe. Tektronix made identical units called TU-7 and 067-0521-00. The 1M1 is used for calibrating the gain and checking the common mode rejection ratio of the mainframe. 500-series mainframes are supposed to have 10 div/volt differential response to signals from the plug-in, and zero response to common-mode signals from the plug-in.