Length varies by model, and I would hazard to guess, by pen. Pen to pen consistency wasn't always the greatest in those days. Remarkably good given the machinery (real skill in the lathe operators) but not what we get with CNC machines.
If you want more information, Custompenparts.com gives rod length so that you can figure out what rod to order for what pen.
I have to admit, at what they charge for the PEEK rods, especially if they are as good as Francis says they are, it doesn't make a lot of sense for me to make it myself. Francis certainly could make them, but he orders theirs instead of making them himself.
I believe Francis makes the MB helical screws for Roger from CPP, so their relationship would be somewhat symbiotic. Peek rods would be a major improvement on the previous carbon fibre rods, where the issue I encountered was the delrin threaded end fixing to the rod was weak. Since CPP came into being, they have been a major game changer in Onoto servicing, particularly the piston seals they supply. I probably service around 100 Onoto's a year and use use Viton "O" rings, rather than cork for the barrel end seal.