i have been in computers before and that was the first time i ever saw a hard drive and cd rom connected by the same cable usually its 2 hard drives or 2 cd roms and i have a pict but i would have to email it to u since i dont have a link to it
Welcome to the SCSI bus. SCSI = Small Computer Systems Interface
SCSI was created as a standard to interface storage to computers, and was not platform specific. SCSI allows 8 devices on a bus, each having an ID of between 0 and 7, including the controller (most times ID 7). It is quite common to have 1,2, or even 3 hard drives on a SCSI bus, as well as a CD.
Mac's pioneered their use in everyday computer systems that normal people use, I believe in the SE or the II, back in like 1987 or so.
What you are seeing is a SCSI ID selector. Drives were either jumpered through a 3-jumper interface (3-bits = 0-7). The selector wheel was a quick way to change the ID, without messing with jumpers.
If you look on the back of an external SCSI cdrom, or hard disk, you will see a selector, although it may not be a wheel-type. Normally they have buttons below and above the number window, which allows moving the number up or down.