And so my Retina Ia yellow filter replacement arrived today. I can tell you right now: procuring a new filter these days is a pain in the proverbial side.
This one is the right one: 29.5mm screw type, for Schneider-Kreuznach/Xenar 1:2.8. If your Retina is a Ia with this type of lens, buy as per above. If you have the 3,5 version with Rodenstock lens, do not buy this one.