I cut the top plastic housing to allow for longer pens on several plotters we had at a prior job. At the time we had a large sheet drafting plotter in the main CAD area and the engineers had the 7475a plotters on their desks. The drafting pins fit the changer except for the length. It was a no brainer to modify so we could use the much nicer drafting pens.
I still have the plotter but no idea where to buy new pens.