I have an Ercona II with the CZJ Tessar lens. There is a deep groove in the front of the barrel that seems to fit a 37mm push-on. I have never been able to find an actual manual for the camera, so I don't know what the official recommendation is. I believe a 40.5 mm push-on might fit over the knurled outer portion of the barrel. I have also read that with at least of some of the Zeiss Ikonta flavors, that rough texture that looks like knurling actually has a thread cut through it that takes a really, really rare sunshade with a female thread. But of course, that's the wrong sex to accept 99.99% of the normally available filters. I also get the impression that 37 mm push-on stuff is rare and expensive because it fits the Bessa II which falls into collector's item pricing.
I have accumulated a few 37mm push-ons, but I find they are pretty un-handy fitting into that groove, and not very solidly anchored, in my opinion. I also have a rig to put Series 6 filters in a push-on adapter, using a sunshade as a retainer. It works, but I had to chuck the adapter in a lathe and thin down the clamping tabs a bit. Mayhaps one of these days I'll splurge on a 40.5/41 mm push-on item and see how it fits. Even the Series type filters and fittings go back so far they are requiring a bit of patience to find.
Someone a while back pointed me to an instruction booklet for a Zeiss Ikonta 523/2 at the David Reichert site that is very close to the Ercona, but it talks about 40.5 mm screw-in and 42 mm push-on and that doesn't quite jibe with my Ercona. It would be great if someone among the 59K APUGgers had some official word on these cameras.