When I started into photography in the late 1950s, Series filters were the only game in town, although the shift to direct threaded filter in various sizes started about that time. Introduction of the Nikon F pushed that trend strongly, as most or all of its initial lenses took the 52mm thread filter. I acquired a complete collection of Kodak Watten Series 6 filters for B&W by 1960. Then as I launched into Nikon equipment, they when into storage for another 15 years, until I bought my first Koni-Omega camera, the lenses for which directly take Series sized filters and come with matching retaining rings, although the Koni system of lenses use Series 6, 7 and 8. At the time, later 1970s, Series filters in like new condition could be had in any type and value for just a few dollars each, so I bought up Series 7 and 8 for my Koni. The Kodak Watten filters were the quality leader, being assembled with two thin plates of optical glass, sandwiching a sheet of filter gel. These were before the lens makers figured out how to uniformly and reliably dye optical glass in batches. Those advancements doomed the Watten filters, which were much more expensive to make.