In the meantime while the search for the Nikkor continues, and since you're nearby, I have a 300mm Caltar II f5.6 (rebranded Rodenstock Sironar N) you might want to borrow/try. They"re both pretty hefty plasmats. It's in older Copal 3. Just a thought.
Daniel, I have a Nikkor-W 300mm f/5.6 that's included in a Toyo kit up for sale. See mjschimpf.com/4X5 for a Lightroom web pre-view. the lens is in excellent+ shape used only a few times.
You might have already found the Nikkor you're after but if not it's on a home-made 6x6 lensboard (translation, piece of masonite). Don't know if that would fit the Ritter. Also I do have a Sinar lensboard drilled for Copal 3.