The KT-73 will fit into most of KT-70/71/76A/76C mounting racks and utilizes the same main connector. However, there are other factors to consider. First, the KT-73 rack includes a configuration module that will store aircraft parameters for the Mode S function. This configuration module is located in the silver box on the right side of the rack (please see photo). In addition, the configuration module has a male connector which can be seen when you peel off the silver tape from the box (see second photo).
The designation of this connector is JKT73-4.
All of the TIS (traffic) interface connections for the KMD-550/850 interface are not located on the main connector of the KT-73 (JKT73-1). The TIS ARINC 429 data is output from the KT-73 into the KMD-540 MFD and then output from the KMD-540 into the small config module connector. The small mating connector for JKT73-4 can be purchased separately however it is more cost effective to simply purchase the entire KT-73 installation kit for this scenario.
In conclusion, when upgrading the KT-70/71/76A/76C transponder to the KT-73 with the intention of interfacing to the KMD-550/850 MFD for TIS traffic, you must purchase the correct KT-73 mounting rack and installation kit.