There used to be an LCD Panel that you put on an overhead projector to project the Mac Screen onto a larger screen, it was for the 128K - Plus and SE Series Macs. That is the adapter for that LCD Panel. It goes onto the mother board and the analog board plugs into it so you wont lose the original video, and then a third cable comes out of the case to the LCD Panel.

The Bd of Ed used to have hundreds of those but stupid teachers would always drop and break the LCD Panel!!!