I noticed my Rolleiflex and cord (Xenar lenses) have a 27mm filter thread, where filters and hoods screw in perfectly. I however once read this was a bad idea, but I do not understand why. Does it cause vignetting? I have a few 27mm filters that I'd love to use.
I'm looking at my Rolleicord TLR with Xenar lens if this is what you are referring to and the lens takes Bay 1 filters or hoods. I think people need to know more about your camera so you get the answers you need.