Here's what I found, and sent to @JeremyClarkson
I've logged into the portal as you and plugged in the serial number you provided into the download search. I sorted by download name and found it on page 5.
This should take you straight to the SSH file for the 15.2.1.5 release: https://extremeportal.force.com/ExtrDownloadDetail?id=a4134000000Dm9AAAS