Industrial networking external multiplexer Adtran TSU 100. We've been told that this is to connect your LAN equipment to a T1 broadband data line at a rate of 1.544 Mbps. This is marked with part number "1200.052L1". We powered this up and it successfully completes the Memory & System Tests and boots to the main menu. We were able to navigate the sub-menus. This includes a 10 foot long cable.
We don't know much about this, if you have questions, please ask.