Sounds like you got a nice camera. I just got a Retina (thrift store find) that seems to be a similar model to yours. Mine has a fixed lens, a Schneider-Kreuznach f2.8/45mm Retina-Xenar.
I have some Kodak Series V and VI filters in my stash and so far none of those seem to fit my lens. The manual for my camera mentions some filters etc. such as the Kodak Retina 32 Lens Hood, the Retina Close-Up Kit Model C (which requires a Kodak Retina Camera Platform, Model C) etc. If you want more specifics let me know.
Mine came with a 46mm lens cap that says Made in West Germany; I don't know if that's original to the camera but it fits nice and snug over the outer edge of the lens. This lens is more recessed than some of the other Kodaks I have that take the Series filters, so it makes me think at least some of the Retinas must have had their own type filters and hoods.