It’s taken quite a while, but I’ve finally tracked down a copy of the Yaffa/Page Crypt of Shadows #3. This is a digest-sized issue, which I assumed it would be given #’s 2 and 4 are digest-sized, but I’m happy to be able to confirm it. The twist to the story (and there’s always a twist with these Yaffa/Page issues) is that it was later reprinted as an unnumbered regular or magazine-sized edition, as per James’ site.
To date I have only come across 4 issues in this series, not counting reprinted issues with the same title or indeed with other titles.