I have a Hard Disk 20SC external SCSI drive enclosure that has a SCSI ID selector switch on back. Because I am using a non-stock IBM DGHS 4.5GB drive inside, I've left the SCSI ID select switch cable disconnected. But I am eager to change the SCSI ID on the fly.
Here's a photo of the drive label, showing the SCSI ID jumper settings:
I believe the precise jumper settings follow what is shown on page 17 of this PDF.
And here's a photo of the inside of my HD20SC (a still image from my PSU recapping video):