Welcome to FPN Alec,
I think the reason that you haven't received an answer to your question is because most here aren't familiar with Retro 51. Of all of the vintage and new pens I have, my Retro 51 Lincoln XL is my favorite daily user. Sorry to bore you with the details, but when I got it several years ago, I had the very same questions you do.
After talking with R51 in Richardson, TX. and Brian Gray with Bexley Pens back east, I finally contacted Schmidt nibs in Germany and talked with their marketing dept.
Retro 51 uses a proprietary Schmidt Nib unit. Meaning that the feed and nib are joined into one modular piece. It is nearly impossible to replace the nib without screwing up the whole module. The Module, however, screws out easily (like a Pelikan) and can be replaced. That said, Schmidt makes the units for R51 in Fine, med, and Broad.
I received a regular Medium nib in my pen and wanted a stub medium, so I started looking for a replacement as you are now. I decided that I could have a Broad nib ground to a stub medium. When I spoke to one of the managers at R51, they didn't have any surplus Broad nibs on hand and referred me to Brian Gray because he used a similar Schmidt nib unit in his pens. Brian informed me that his was different and suggested contacting Schmidt directly, which I did.
The marketing guy at Schmidt was very accommodating and offered to send a Broad Regular nib unit to fit my pen. He surprised me by doing this at no charge (not even postage). I then began to look for a Nibmeister to grind it to my specs and found that most will only work on gold nibs, not stainless steel.
I was determined, so I read a few instructional threads on nib grinding and did it myself. The result was incredibly satisfying and surprisingly easy. And best of all, I got EXACTLY what I wanted!
I have since learned that Danny Fudge (http://thewritepen.net/) will grind SS nibs and is very reasonable and quick by comparison.
I hope I've answered your questions and help save you some time. I cannot locate the name and contact number of the person I spoke with, but here is the Schmidt web address -http://www.schmidtte...eme/c_1_3_1.htm
Good Luck in your endeavor! Let us know how it turns out-
Edited by Hooker56, 05 January 2016 - 04:03.