In August 1967, while the College Eye Task Force was still based at Udon RTAFB, another prototype EC-121 variation began operations testing new equipment as Detachment 2 of the Tactical Air Warfare Center. Known as "Rivet Top", this modified EC-121K (later redesignated EC-121M) carried the QRC-248 newly installed in the College Eye aircraft and also had electronic interrogators capable of reading 2 additional Soviet transponders, the SRO-1 and SOD-57. Its electronics were custom built rather than off-the-shelf. Its most important upgrade was the top-secret "Rivet Gym" installation. This consisted of the addition to the crew of Vietnamese-speaking intelligence specialists manning 4 voice comms intercept stations able to monitor all comms between the MiGs and their GCI controllers.