hi! My guess after looking at this and doing cursory research is that that is exactly what this is. This release was added in 2003, which is a year before I started!
URL relationships were added together with the other "Advanced" Relationships, added a long time ago.
While thinking on this, I vaguely remember there being some ASIN-links that would "magically" show up given the right circumstances, something similar was Barcodes added making the coverart for ASIN (ASIN used to be the only way coverart was associated with releases, there was no CAA) that correspond with that barcode "invisibly" associated with the release, no url to show for.
It could very well be that the url's history was eaten and mangled by one or several of the site-wide schema changes that changed quite a lot: the the NGS (next generation schema), redesign the entire site, old predefined insertions of edits. etc.
Given that this is such an old release there are many ways that this could have gone blipp.