We couldn't find the page that was requested:
https://www.gbps.org.uk/information/sources/files/staff-handbooks/Rg111%20Instructions%20for%20Indoor%20Officers%20Dealing%20with%20Registered%20Postal%20Packets%20(1925).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.