Found it! Filter adapter for Retina II
A while back, I purchased a few Kodak Series VI press-on adapters and B&W filters for my Retina IIa (Schneider Xenon lens) that worked just dandy.
When I got my Retina II with a Rodenstock Heligon lens, however, I discovered that the knurled ring at the front stood a little proud of the rest of the lens casing. Putting a Series VI adapter over the snug fit on the front, it hung loose. Adjusting the press-on tines didn't work.
I put it in the back of my mind to find a screw-in adapter instead. Months later, today, I finally took a trip to Kenmore Camera in north Seattle and worked my way through their boxes of filters and filter adapters between 28mm and 30.5mm. Not a lot there in the 29.5mm range, but with patient help from an employee at the counter, we found a Tiffen #509 screw-on adapter. Bingo! It takes Series V filters, and I quickly found a fine K2 to put in it. Here's how it looks. Now for some photos in the low fall sunshine, while it lasts.