Mullen Extender Boards
This was a well designed S-100 extender board that allowed you to examine any S-100 board while present in the S-100 bus.
It had a basic TTL logic probe onboard.  There were at least four versions of the basic board -- of increasing sophistication as to the logic probe. Here are two:-

The edge connector at the top of the board is fairly fragile, if you are not careful to pull an S-100 board straight up out of the socket you can crack the sides of the socket.

Other Mullen S-100 Boards

Extender Board     Relay Board