While the web-ops group did put quite a lot of effort into trying to avoid broken web links, unfortunately it seems we didn’t catch all of the variant web addresses that some pages have.
The canonical form of the address of pages in the old Drupal site was <https://britastro.org/node/25856>, where 25856 is the page number. I believe all these canonical URLs should correctly forward to the right page in the new site, including this one.
But as you have discovered, unfortunately the “node” bit could sometimes be substituted with other words in the old site, and many of these alternative page addresses no longer work.
So, if you’re trying to fix a broken link, you can generally find the page you’re looking for by changing the broken link into the canonical form and seeing where it redirects to in the new site.