This is an interesting piece
of hardware, it appears that it's a memory (and other register) viewer.
The board with all the LEDs is hard wired between the cartridge and
the system, and whenever a data path or register is set high, the
appropriate LED lights up. The other little board has a series of
DIP switches that allow you to disconnect each 'pin' of the cartridge
from the system individually.
This hardware was built
by an Atari employee to help debug Lynx games. I believe it's a one-of-a-kind
as it was hand made, but if it was really useful, maybe more were
Here's a close-up of the
swithes and the LEDs: (the board with the switches also has
logic-probe points for every pin of the cartridge.)