The Tektronix 040-0652-xx is a mechanical kit to make a blank single-bay plug-in for TM500-series mainframes. The 040-0754-04 is a double-width version.
The kit includes a printed circuit board that interfaces with the mainframe and has free space for the end-user to build custom circuitry. Drilling the blank front panel and installing custom controls, indicators, and connectors is typical with this sort of blank plug-in. The manual gives guidance on interfacing with the TM500 mainframe, particularly regarding power supplies.