Deciphering "wee old lady" library book code

Georgia Grainger, a Scottish librarian, began a fascinating Twitter thread earlier this week:

Turns out from the ensuing comments that this is a rather common practice. A dot on page 50, a circle around the number 40 on page 40, a little "P" on the title page, other librarians and readers started to chime in with the codes they have seen. It seems to be mostly elderly patrons and those reading from sections like Romance, where one can burn through a lot of books with self-similar subject-matter.

See the entire Twitter thread here.