I recently looked through an older print from around 1800 and noticed numbering of the pages in addition to the regular page numbers. The numbers were both Roman numerals and letter-number pairs (C, C2, D, D2) intermixed.
What is the purpose behind this?
It was more of a guide for the workers of the print shop than for the reader.
In order to print a book in a format like in-8, you have a folio and fold it twice, one vertically and one horizontally. Like that, you have a printing surface a quarter of the size of a folio, and you can print 8 pages using a single folio. Take a folio, fold it into 4, and mark each of the pages: 1, 2, 3, 4, 5, 6, 7, and 8. Now, unfold it. That would be the distribution of the pages in order to print them.
The lettering A, B, C, etc is also there to help the printers. The printing order could be fairly irregular if you had a printing shop with more than one printing press, so marking the fascicules of which the book is composed. That way, when the time to bind the books comes, there would be no problem: you arrange the fascicules in alphabetic order.