Filters on Ikonta 521/16 vs 523/16

I have a Zeiss Ikonta 521/16 with the 75mm Novar that takes a Series VI pressure-fit filter holder in a circular slot on the front of the lens. I recently purchased a 523/16 with a slightly different looking 75mm Novar. The filter holder will not fit into the circular slot on this new lens, although it does appear to be the same size. Does anybody know what's going on?

No, but if you take a ruler and measure across the lens you should be able to find a series adapter easily enough that will fit on the outside by pushing it on, and will take a series filter. I had the same issues w/ a little 6x4.5 531a Ikonta once (I think that was the camera), and it was just easier to go w/ the slip on adapter than to find anything that fit into that inside groove you mentioned. Zeiss made filters for just that purpose, but they are special to each camera and cost a pretty penny.

Ok, I measured and there is a .5 to a full mm difference. Is the 523 one of those where the lens has the male end and the filter the filter the female end of the screw relationship? If so, I have a whole bunch of looking to do - I really want to make this 523/16 my everyday shooter and the 521/16 into a Christmas present for my son-in-law. Umm, how pretty of a penny?

I have several ikontas, they have a double system to fix acessories. there is a 32mm clip-on type, which slides into the groove around the lens and 35.5mm screw-on type, which screws to the outside of the lens. the knurled surface around the lens is a kind of thread, even if it does not look like one. try it out.