No specs; a custom private-labeled beast.

I wondered about threads vs ridges, but I was able to thread a Sonia fine-pitch threaded cap on a little bit, more than once too. Lotsa squawling and other unpleasant noises. I decided to just thread it on far enough to stay on, to keep me from messing up the front element.

I am planning on rear filters. It's has an interesting back end. The outer diameter is unthreaded and looks like it's very thin, as though there were a space between the outer diameter and the next concentric layer around the glass. Outer measures 39.96 mm (say 40), and inner measures 36 mm. I'm not sure if there is enough room to fit a 36 mm push-on adapter, if there were one available (and it would probably be Series VI). I am looking at some 43-49 mm step-up rings which have an inside unthreaded diameter of a tiny bit over 40 mm. This would allow me to use some modern filters instead of aged, damaged or scarce Series VI. Most of my S-VI ones are bubbling, delaminating, Newton-ringing or whatever their problem is.

Could be that the lens retaining rings are why the lens barrel is threaded. It was intended for indoor studio use. I imagine it would be preferable for the photographer to do his/her thing with vignettes, etc. much more easily from the front than poking around inside the camera (long roll 46mm), if that's even an option with a roll of film in there.

I could also use unmounted Surplus Shed aerial yellow or red filters with the help of a machinist, but the easy route is something I occasionally try to remind myself to look for (I may ignore it once I find it, however).