*My* guess is that maybe he didn't keep proper running source notes as he was working, and didn't have time to get them coherently reconstructed before the book entered production, so added the web address as a stop-gap and cobbled something together there in time for publication.

yeah, something like this is what's most plausible to me; my first thought was 'the editor completely forgot to ask for this until five minutes before it needed to go to print', because I've done that sort of thing myself. I do not think it is a cost-saving measure or anything like that.

That sounds about right, thanks.