The TU-7/1M1/067-0521-00 is a letter series plug-in used for calibrating 500-series scopes. It is a multifunction replacement for Type P. It's used for calibrating the mainframe's step response and gain, and checking dual-trace operation, common mode rejection, and power supply regulation. (500-series mainframes are designed to have 10 div/volt response to differential signals from the plug-in, and zero response to common-mode signals.)
It contains a transistor multivibrator, whose output is sharpened through Tek-made Gallium-Arsenide diodes (later production used silicon), a dual-trace circuit using 12AT7 and 6AL5 tubes, and various load resistors. It does not have Type P's short operating life, because it does not use a mercury switch.
This instrument went through several names. At its 1964 introduction, it was the TU-7. Some early Tek-internal instruments for design, manufacturing, and calibration had TU (Test Unit) nomenclature. (Other examples include the TU-?? Variac, the TU-?? dim-bulb fixture, and the TU-50 sine/square/pulse generator.) In 1965 it was renamed Type 1M1 to fit into the letter-series nomenclature like its predecessor. Later still, the 067- calibration fixture naming system took hold and it was rechristened 067-0521-00. The instrument was also revised once - more pulse frequencies - and the name upgraded to 067-0521-01. It does not appear - under any name - in any Tek catalog I know of, but the 1964 catalog _does_ list a cousin: Type 10/11M1, for the Type 647 scope.