Kodak Cine Ektar Filter Sizes

I have Kodak Cine Ektar 15mm f2.5 (made 1955) and Kodak Cine Ektar 25mm f1.9 (made 1949) lenses, both in S Mount. Does anyone know their respective filter sizes? Kodak retaining rings (impossible to find now) used ID numbers that give no hint of size. The APUG community is my last hope for an answer. I received no replies from cinema equipment dealers or the cinematography blog. I need filter size to attach ND filters. Not necessary when Kodak made reversal Plus-X, but now only make Tri-X reversal.

You can use Series filters. Adapters are either threaded or slip on. A Series lV filter is
about 7/8" diameter.I don't have a V to check

The adapters were available literally in dozens of sizes. Slip on were measured in inches and threaded in mm.
There were some made for dedicated lenses.

I have both lenses.

I was able to get a 27 mm to 46 mm step ring to go a couple of turns into the 25/1.9. This so I could reverse mount it for use as a macro lens.

A Tiffen "38 F 6" Series 6 adapter ring screws right into the front of the 15/2.5. John, this lens can't use a slip-on filter holder, its aperture scale is at the very front. A slip-on would cover it.

Thanks Dan, I was able to screw a 35mm filter for a summar lens into the 25mm. So I ordered a 35 to 42 step up ring, since several of my movie lenses use 42mm filters. I bet one of my other 25mm Ektars takes a 27mm filter size. I have several 25mm Ektars, each with a different filter diameter. The one I am interested in now is for S mount. I will try the Tiffen series 6 for the 15mm. The manual for my Kodak K-100 covers John's reply. The problem is that the adapter rings for the various lenses go by a code number and not by mm. My suspicion is that Kodak was catering to the cinematicaly unsophisticated customer and hid much of the technical data in order to force the customer to buy Kodak products. And of course nowadays you can not walk into a camera store and buy Kodak adapters.
Thank you, John and Dan, for your replies.