I've inherited a Retina IIS, which is the fixed-lens variant of the more common IIIS. I've not been able to find a manual for it, although the IIIS's manual is fine for almost everything.
What that manual doesn't say is what the filter thread is for the lens. I want/need a yellow filter to make it a bit more habitable on bright days. The lens is made by Schneider-Kreuznach and describes itself as a Retina-Xenar 45mm f/2.8. It's in a Synchro-Compur shutter.
I know the thread is slightly smaller than 30.5mm because I have a bunch of 30.5mm filters. I suspect it's 30mm but I'd like to avoid spending money on a 30mm filter only to find it's something odd. (I presume it's metric since the camera is German, but even that is not certain perhaps.)
So, does anyone know definitively what the thread is for this camera?