It's for the Apple DOS card in the 6100. The two ends on the right plug into the socket on the DOS card and the video output on the Mac. On the left, the black one is to connect a PC joystick (I'm not sure why, but pretty much all Mac DOS cards had a joystick socket) and the last one is the passthrough for the video -- you would connect your monitor there.

Edit: More info and pics here:

http://homepage.mac.com/olivers/DOScard/MonCables.html