TM500 system
The Tektronix TM500 / TM5000 is a modular test equipment system offering a variety of plug-in modules that fit a dedicated series of mainframes with one to six module slots. The plugins are not compatible with other Tektronix lines such as oscilloscopes.
In addition to the modules that appeared in Tek catalogs over the years, there were probably a hundred or more that Tektronix designed and used in-house on their production lines for test and calibration of their products. These in-house units usually have an 067-xxxx-xx number.
There were also several other companies that made plugins that fit into the TM500/TM5000 mainframes. The most common non-Tek plugins seem to come from Pulse Instruments, Spectracom, General Optronics, Lasertron, Metrotek, Xetron, Ball/Efratom, Digital Audio Corp., and Ballantine.