"Monocular"....as in one-half of a
binocular!!! Ohhh, I used to have an old monocular, a 7x35, made back in the 1960's...it was fun to use.
I have a few small Ednalite filters in cases and original paper boxes. Ednalite was a popular filter brand back in the day. YES, those were called "Series" filters, and they fit into a holder, with a front, screw-in retaining ring. The "series adapter" was made in multiple sizes, like V, VI, VII, corresponding to diameters. MANY 35mm cameras of the 1940's and 1950's had relatively small-diameter lenses, which meant that a 39mm diameter (something like that, not sure really, but by today's standards, the series mount filters were "tiny") filter would cover the front lens with PLENTY of room to spare. So....filter makers made thread-free filters which were used in "Series" filter holders.
This will show how the Series filters and their holders were used:
Series VI filter holder - Google Search
Good score on the bin full o' camera stuff!