Digit 5 as "y" with tilde, and spurious "quire" mark on f57v

[Koen G]

It's just one of the ways to write "g", and I'd say a rather common one. Drawing a separate horizontal line that connects to the next letter is probably much easier than making a circle with the pen. This is also the reason why the line starts at the left vertical stroke but extends way to the right: it serves both to close the top and to connect to the right.

This is, by the way, the exact same way of writing "g" as the one used by the month name writer in "yong", and I bet these were written by the same person.