I have found step-up rings that are a 'wedding night fit' to the focusing ring and then epoxy them in place - done this with several lenses that are weird sizes or have series threads instead of filter threads. With an extra step-up ring or two I can mount my 'standard' 52mm filters. Looks really weird but finding slip-on filters for these things is hard - as opposed to loosing them, I had a yellow/orange/red set with a lens-hood for my Nettar.
I don't know the size for the 105mm, my Nettar is a 6x6 with the 75mm Novar.
They are great cameras. The pictures they take will put any and all 35mm shots to shame.
The 32mm filter fits my 6x6 Nettar 518/16 but my 6x9 520/2 has a Zeiss Tessar 10.5cm lens which is bigger so I must change my answer to I don't know which isn't much help!
I have the same lens on a 6x9 Zeiss Ikon Ercona and the filter is a press on 37mm. They are hard to find and, when you do, pretty pricey. I am holding filters in front of the lens right now but I think it would be nice to find a press in filter step up adapter. If anyone has ever seen anything of this sort, or knows someone skilled enough to build one for a price, I am certainly interested.
I think I remember seeing a device for holding Cokin square filters in front of a digital camera without a lens filter thread. It attached to the base via the tripod thread. Should be a fairly simple thing to make.
That is a great idea. But does it reach out far enough to use with a folder? When my 6x9 bellows and lens is deployed the bottom tripod socket is 5 1/2 inches (about 14cm) from the front of the lens. I have another tripod socket in the bellows cover. It is not centered with the lens but it is still about 2.75 inches (7cm) from the lens.
Has anyone used one of these? If so, will it reach in my situation?