British Museum Copyright Stamp (Seal)

Dear Rich: I was wondering if you could give me some information about a stamp. I have attached a picture of it. It is from a book in the British Museum. I'm trying to figure out what the number 20 on the left hand side means. Also do you have any idea what year it could be from? As I know the stamp changed some time between 1960's and 70's, from the old lion and unicorn stamp. Sorry, but we are unable to locate any information about the stamp, including any similar stamps -- we used Google's reverse image searching feature. We've posted it (left) in case other readers can fill in the gaps. We're probably off-track, but if it helps, the museum offers a 6-volume, geographically arranged, published catalogue, Catalogue of Seals in the Department of Manuscripts that covers all types of 'seals.'