I had a couple of old Rapidograph pens laying around and started playing with various nibs. Thinking German pen why not try a German nib I test fit some Pelikan 200 nib units--- they matched the threads and screwed in fine but were too long. The nib extended out too far for the pen cap to thread on. I tried both 200 and non-M&K 120 nib units and both threaded in fine. I did find an inner white plastic cover inside the pen cap but even with this removed the Pelikan nibs were just a little too long. (I reset the nibs and feed as far back as I could.)
Then I remembered reading something about using Esterbrook nibs in a Rapidograph pen and victory! Recent discussion on FPN mentioned using Osmiroid nibs in Esterbrook pens so that looks like a fruitful idea to try in the Rapidograph pen too. The hybrid is a nice quality piston filler about the size of a MB146 and Rapidographs can usually be found very inexpensively.
I would like to fit a pelikan nib though and would appreciate any ideas to do so.