Industrial networking external multiplexer 1.544 Mbps Adtran TSU 100. We've been told that this is to connect your LAN equipment to a T1 broadband data line. This has P/N "1200.052L1". We powered this up and it successfully boots to the main menu and completes the Memory & System Tests.  This has a 10 foot cable.

We were able to navigate the sub-menus but we don't know much about this, if you have questions, please ask.