If ou plan to toy around with several old lenses with different filter sizes, I strongly recomend to go for the old series filters with holders. Series filters can be found in a few different sizes, and the holders come in every imaginable size, both slip-on and screw-in. The idea is that one set of filters can fit different holders for different lenses. I had a lot of trouble and expenses collecting slip-on filters. But then I was given a set of Leitz series VI (roman 6 that is) filters and discovered the advantages of this system. Now I have holders for 31,5mm, 36,5mm and 42mm slip-on and 39mm screw-in. I also have different lens shades that screw into the holders (replacing the filter retaining rings. Remember that lens shades are an absolute must for old uncoated lenses. There also are series V and VII filters (at least), but I think series VI is the best alternative for 6,5x9 and 9x12 cm plate cameras. They are 41,5mm in diameter, and the very common 42mm slip-on size is perhaps the largest ou can go with these. If you need bigger holders, I think you vill need to go for series VII. American e-Bay is the place to look, as the series system was more common in USA than in Europe. You'll easily find several filters in one place, so you can save postage and time. Good luck!