What is the difference between the AN/USM-719, AN/USM-708 and the TR-420?

All three test sets are based on a common design and are digital testers with software-based test packages.  The TR-420 is the commercial, off the shelf version of the test set and includes IFF Modes 1, 2, 3A, 4, S and Mode 5 Transponders and Interrogators.  The TR-420 also includes EHS, ADS-B and TACAN functions. The AN/USM-708 and AN/USM-719 were was designed for the U.S. Navy and add additional Comm/Nav/VSWR and shipboard tests.