Many vintage cameras had a lens without a filter thread. Manufacturers such as BDB, Leech and Actina created push-on filters for users of that type of camera.
These filters, typical with an aluminium mount, push over the lens and hold in place with a small spring clip.
They came without a glass filter and had either a plastic or an aluminium retaining ring to hold the glass or resin circular filter in place.
They’re sold with two measurements in millimetres. The first number is the lens diameter, and the second is the added filter diameter. Usually just the push on size was engraved on the rim of the holder while both numbers appear on the box – eg 40/38.
There used to be a wide variety of glass filters that can still be found in second hand camera shops or auction sites and resin or gel ones can be easily cut to size from cokin or Kodak Wratten style filters so there’s many options to hand.
There are a selection of these push on filters for sale here: https://www.photographyattic.com/tag-355