I have a Series IV filter and have been unable to find any information on it. All the information I can get refers to series V and above. This series IV is much smaller.
Could it be for a movie camera perhaps? Could it be for a much earlier camera like

The Kodak Series IV filters (13/16) were used with the Series IV adapter rings and were used mostly for movie camera lenses although some of the Kodak Series IV adapter rings were made to fit Argus C3 and C-2 cameras with Cintar lenses and Leica cameras w

They fit the Stereo Realist as well.