I have an Ercona II, an East German Ikonta with CZJ Tessar, that works with 37 mm push-on filters. It is a bit tricky, as they go down into a deep groove in the lens barrel. It looks to me as though a 40.5 mm push-on might work over the outside of the knurling too, though I've not tried it. So far I have some 37 mm push-ons, and have also lightly machined a Series VI push-on adapter to fit in the groove and use series filters already in hand.

Unfortunately, 37 mm push-ons apparently fit the Voigtländer Bessa II, which adds some collectable value to the prices, especially the Voigtländer branded ones. I've not found them as common on ePrey as i had hoped. I can't remember when the transition occurred, but it seems series filters went the way of the dodo several decades back.

I have also read somewhere that the knurling on the Zeiss ikonta actually has a screw thread as part of the pattern and that there were female threaded 40.x mm adapters or filters, but I've never seen anything to confirm that notion. And I don't know if all vintages were that way; my Ercona appears to be a straight knurl, no thread.

As someone upthread noted, the camera doesn't close with a filter in place, a compromise for compactness when folded. They are really nice cameras -- even my East German imitation (which actually came from the original Zeiss plant).