Thanks guys and a Happy New Year to you too!
I'm totally thrilled with this Cosina. Of everything that limits me in producing better macro shots, I feel this lens is by far the least of my worries. If you're a bit like me photographing-wise, it could be the only macro lens you'll ever need.
The 1:1 macro adapter that comes with the Cosina happens to fit quite nicely in the front rim of the 50mm f/1.8 Ai-S MkIII. You'll have to hold it in place with your hands though. For me this is not a problem because I focus manually anyway, mostly by moving my head back and forth (with the camera at my eye of course
Same thing with the 50mm on the (adapterless) Cosina. As the extending part of the Cosina has the same diameter as the adapter, you can reverse-fit the 50mm to it by its front rim and handhold it there while shooting. You can get very close to your subject this way, though I've found it to be pretty hard to nail the focus or to get a proper DoF. Also, forget about corner sharpness, and you have to love the occasional vignette. Anyway, for some shots it does work.
Perhaps there are also adapter rings available to make the step from the 49mm thread of the Cosina to the thread size of the Nikkor, to free you from having to handhold the second lens.
Hope this helps.
(Oh and feel free to PM me of course!)