You probably mean a thread for filter? It's probably a metric fine thread (called sometimes "optical").
Goerz, and many lens makers back in the day, did not design their lens barrels to take the threaded mm-size filters commonly used on small-format cameras. Manufacturers like Kodak, say, would offer press-on, friction-fit filter rings that held unthreaded Series-size filters with a retaining ring. (Your lens would need a Series 8 filter.) I doubt you will find an adapter ring of any sort threaded to fit the front of your Dagor, though. Clamp-on filter holders like Lee or Cokin will probably be needed.
Or you can have a custom machined adapter, or add a custom machined barrel flange threads.
I use series press on filter holders on my dagor lenses. you could get a press-on and and step up to a mm filter size.
Thank you guys, now I know!
Tomas
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