I had a customer who had a Penman set, a FP and a BP. The ballpoint was dried up and is no longer available so I had to make something up for him.
I normally install a metal nose cone but because this section was not screwed in there would be no way to install a spring to keep the refill where it belongs so I had to come up with another solution.
The original refill just pushed in from the nib end so I ended up using a Bic stick refill. I had to machine a new sleeve that went inside the original section and then drill it out to take the Bic refill.
I know it is a Bic but at least this matching pen set is usable.