The slip-on filters are designated as A37. The Zeiss Ikon filters come in small dark brown bakelite cases. The filters have three prongs,which grip the side of the lens.
I've been able to get a small collection of Zeiss Ikon filters in various sizes for a small amount of money. I don't think that I've ever paid more than $9.99 for a filter.
Although the filters are now at least 50 years old, they're optically fine. Almost all of them will need to be cleaned. Clean them like you would a lens.
Now, regarding a Zeiss Ikon 6x9 folder. You have several options. First, think whether you would like a camera with a Novar or Tessar. The Novar (a triplet) is a fine lens although the Tessar should perform better at larger apertures.
Next, do you want a coated lens or an uncoated lens? I have had no problems with using an uncoated lens with color film. The more important factor is whether the lens is clean and not scratched.
And finally, do you want rangefinder focusing or zone focus? If you'll be shooting portraits and such, then go for a rangefinder. A zone focus combined with a handheld rangefinder is perfectly adequate too although less convenient.
All 6x9 Zeiss Ikon cameras have body numbers that end in "/2."
Among zone focus cameras with Tessars, there is the 520/2 (prewar) and the 523/2 (postwar).
The Mess Ikonta 524/2 is a very impressive camera -- very well made. It has an uncoupled rangefinder, so you always have to remember to check the lens so that it's set to the correct distance.
And finally, there is Super Ikonta 6x9. The most-coveted model is the one with the coated Tessar. Again, the camera with the uncoated lens is excellent.
Some of these bodies have shutters that are released with the lever on the shutter. Early Super Ikontas have a small plunger near the shutter. Others have a release on the body, and it's always a left-hand release for the 6x9 models.
I have some mini-reviews
of the various models on my site. I haven't written reviews for every model, as I just haven't gotten around to it. I tend to write these in bunches.
The most important factor of any folding camera (regardless of brand) is to get a camera that hasn't been dropped or abused/used hard. Some photographers are very hard on their gear, and it's their camera. You just don't want to be the next person who owns it.
My favorites are the Super Ikonta 530/2 (uncoated lens), the Mess Ikonta 524/2 (coated lens) and the Ikonta 520/2 (uncoated lens) ... in that order.
The Ikonta 520/2 is the best value, I think. The prices for the Mess Ikonta and Super Ikonta have been pushed up by collectors. The Super Ikonta with the coated lens goes for ridiculous amounts, making the Mess Ikonta look like a bargain considering that it has the same lens.