I suppose that you mean a close up Rolleinar for a TLR since you mention "Heidosmat" which is the finder lens of the TLR. This is always a set of two Rolleinars. In the upper one there is a glass wedge for compensating the parallax between the two lenses (the so called "Rolleipar Keil"). This wedge has to be inserted in correct orientation, otherwise you'll have a finder picture different to that on the film. The second lens has no wedge and is for the taking lens.
Yep that's the filter I'm talking about. The compensating wedge. It's got a slight rattle when I shake it. I'm just waiting for someone t say, "Then don't shake it."