Recently got a Faber Castell Presto 40 felt tip pen, NOS with box, from way back when just the refill was throwaway, not the whole pen. The only issue is that the refill is blue, while the pen is color coded black at the ends. These were also available in blue, red and green. Not wanting to use up the refill (which still works) I cut down a partly used Bic black ballpoint refill, thickened with a bit of tape and it fits.
So many vintage pens face the challenge of workable refills. Good job on the hack!