I don't have a Bellows K, but did just get a mint Bellows A, and if the K is made like this you may be able to get the lens off.
First, is the lens release tab like the one in the lower right of this picture?

If so, does your bellows also have the round knob on the side like above? Unscrew that knob and the bellows will slide back from the lens flange, so you can see inside the mount from the rear. In this picture, the yellow line points to a leaf spring that forms the lens release mechanism. Depending on the lens you mounted, you may be able to see it like this:

I was able to insert a flat blade (or tiny screwdiver) and pull this leaf-spring back so the lens would release. Of course, as soon as the lens turns it hits the tool, but by then you can remove the tool and the lens is free.
I hope your K bellows is made like this one.