I was sorting out my loft a few weeks backs and I found an extremely old cover for a Book 40. It must have dated back to the eighties or older.
I can remember them, but I can’t for the life of me recall what they were used for. Highly unlikely that anyone here would know, but does it ring a bell with any ex-police members?
If you haven’t found the answer to your question yet, you could maybe send an email to the Met Police Museum - their website gives the email address. If you click on the Contact Us section, it also has a contact form you can complete to ask a question.