Bumping a pretty old thread here, but since I happened across it: This is a PowerPort and its purpose is to siphon power off the drive port and provide it to the ThunderScan, since the ThunderScan needs power but the 8-pin printer connector doesn't carry the power it needs. So, I'm pretty sure (haven't tried it yet) that you pass any external drive through this, plug the 8-pin connector into the printer port on the Mac, and plug the ThunderScan (or some other power-hungry peripheral) into the serial port on the adapter.
My source for this is a bit in the left column here in the June 1, 1987 issue of InfoWorld:
http://bit.ly/OCW4zg