"Shipping fixtures" are the enclosures that were used by Apple dealers when a drive needed to be sent in for repair. Customer brings in drive, enclosure is replaced at the dealer, drive is sent to the repair center, gets returned, customer's enclosure is reinstalled.
For a while it was easier to get a shipping fixture on ebay than a regular drive in good condition. Here it looks like someone just needed a decent box for their Seagate ST225N SCSI drive. And now it's a collector's item ;-)
Reminds me of a compact Mac I fixed 20 years ago. The owner broke the CRT neck trying to replace the hard drive. I couldn't find a suitable replacement CRT at the time, so we used a green one from a cash register. The guy was happy with his green screen Mac. And a few years later, I was told this thing showed up on ebay, "Rare Prototype: ..."