We couldn't find the page that was requested:
https://www.gbps.org.uk/information/downloads/files/official-documents/Report%20of%20the%20Postmaster%20General%20on%20the%20Post%20Office%20No%2012%20(1866).pdf
We've checked for it in our site catalogue, but unfortunately that one doesn't seem to be listed.
However, if it's a genuine variety of GBPS page (possibly we re-catalogued it and changed the reference) it looks as if it might be found somewhere in this section – so with any luck, the links in the menu on the left will help you to recognise it. Hopefully you won't need to use a magnifying glass! Alternatively, you can try looking it up in the site map.
Reports of missing GBPS pages are welcome, as they're almost always constant flaws. Please with the details.