I have the Rolleisoft 0 and 1 filters for both bayonet 1 and 2, and the earlier Duto-filters for the Rolleiflex Standard. They are all of the Duto-type with concentric rings that superimposes a weak unfocused image onto the focused image, creating a nice softening effect. The effect is depending on the aperture, and it's not recommended to stop down more than max f:8.
I haven't used them much. but here's an example taken with the Rolleisoft 0:
A portrait taken with a 1938 Rolleiflex Automat, Aperture f:4, Light green filter and Duto 1.