Basic Old Lens Question

I imagine this has already been covered, but I'll ask it again. I've been offered a Gundlich Turner-Reich Astigmatic 13"/21"/28" lens at a REALLY good price, I think ($100.00). How does this lens work? Do you disassemble it & put it together in a different configuration for the differing lengths or what? It's in an old shutter that seems to work but needs cleaning. The lens elements are clear with just a little haze.
Looks like it's either a pain to use or it might be an interesting find. Thanks for any help or direction.

13" - Both elements.

21" - 21" element on rear of shutter.

28" - 28" element on rear.

These lenses sometimes had centering issues. A good one is a pretty nice lens.

I have this same lens. Each group is marked with its independent focal lenth. With both groups installed in the shutter, you have the 13" FL. If only using one group, it is best to have it mounted on the rear of the shutter or you might experience some vignetting. Most of the literature I've read suggests using a yellow filter to enhance sharpness, and I've found that to be the case. There is a small amount of focus shift as you stop the lens down, so you need to refocus the image after stopping down to working aperture. Trust me...at f22 that ain't easy! I've gotten very good results from the elements combined. Less so with individual elements, but I have not used it that way very often and I suspect myself of forgetting to refocus each time I stop down.

Thanks Michael for pointing out the focus shift and filter info. My brain must have been in a different gear.