I sent Mark an email, but it looks like a signature reference. Early books are often unpaginated, but the signatures have letters assigned to them, so the person putting the book together knows in which order the signatures go. Starting with A through Z and starting over with AA if necessary. A signature contains 2 physical pages (4 paginated pages) if the book's a folio, 4 / 8 if it's a quarto, etc. The physical pages of the signature are numbered also. So F means the F (or 6th) signature, iiij is Roman numeral for 4. (It's a convention to write the last "I" in a Roman numeral as a "J".) Not sure about "b", it could refer to a column (a and b in a two column page), but perhaps it means the other side of a physical page. Anyway that's what I came up with. We can wait and see what it actually is.