The L Plan 20/0.4 has RMS thread (when I bought it I was thinking it was a Nikon L Plan CFI60 serie with M25 thread and 60mm parfocal) and 45 mm parfocal with a WD of about 10mm. Few weeks ago I bought a old Vivitar 200mm M42 lens (inexpensive on eBay) with the idea of use it as a tube lens for a Nikon Plan 10/0.25 infinity objective that I have. I decided to use the Vivitar as tube lens of the L Plan 20/0.4, you can see the result on the thread that I mentioned above.
Here a picture of the camera + Vivitar 200mm and some M42 spacers (the camera in the picture is a old Canon 500 only for illustration because I was using my Canon 450D to take the pictures):

To use the camera + Vivitar lens on my Nikon Labophot I removed the trinocular head I mounted the camera + Vivitar lens on the Labophot stand.
Here a picture of how I plan to use the objective with a horizontal setup that I am building:

Rogelio