So it turns out that over these past few weeks my go-to camera has been my Zeiss Ikon Nettar 517/16 with a Novar-Anastigmat 75mm 1:6.3 lens. Now this is all well and good, except that the lens has no threads whatsoever with which I might attach filters. A bit of Google-fu revealed that the Nettars with a Vario shutter (which mine have) use slip-on filters[1].

So how difficult is it to find these beasties?

Oh, and on an almost unrelated note—was there a 6x4.5 version in the 517 series?