I suspect that it must be a header-wire which will probably not get used anyway, but I'm just wondering what is it for.
Everything else about the led adapter works though. The power LED flashes OK, the HD led flashes OK, The reset header/button works, the standby header/button works...Perhaps it's an extension to HD led, in case you want to connect it to pins of a SCSI controller, in order to get SCSI activity on the same HD led? Just guessing...