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

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