The J-4024/U os the interface between the TSEC/KG-84A and teletypewriter sets in GRC-106 communications shelters.

According to the U.S. Army  manual,  "the TSEC/KG-84A is a dedicated loop encryption device (DLED). It is used to encrypt/decrypt teletypewriter and digital data traffic on dedicated links. It can be used for local or remote operations and will operate in the DUPLEX and the HALF DUPLEX modes. Unlike the TSEC/KW-7, which it replaces, the KG-84A will operate in the DUPLEX mode without an additional device. It can operate in the TRANSMIT and RECEIVE modes at the same time. When used with radio teletypewriter sets AN/GRC-122, AN/GRC-142, AN/VSC-2, and AN/VSC-3, the Installation Kit MK-2488/G must first be installed.

Interconnecting Box J-4024/U interfaces the KG-84A with Teletypewriter Sets TT-98/FG, TT76/GGC, TT-122/TG, and TT-4/TG. For complete wiring details of the KG-84A in radio teletypewriter sets, see appropriate technical manual. Do not attempt to use the KG-84A without Installation Kit MK-2488/GR."

Photo courtesy: kcgonzo(at)aol.com. 

Photo courtesy: kcgonzo(at)aol.com. 

View of chassis. 
Interconnection detail
Nameplate detail.
All images in this table via E-bay

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May 5/08