From what I am able to gather via some pickers on the street; with C meaning Cercle, Culb or Casino, the variety of books from just Jost, alone, run from A to Z combos.... giving the customer a good idea what a finished product would look like on stock denoms/monetary units all in one glance...

So far we know that Caro books and displays depicted a crown 👑 on their reverse with mostly French samples and, the SPA from P&P that I submitted to CG with a blank reverse is also known with the 👑 only along with stock inventory codes in combinations of letters and numerals....