These are the
removable pens for the 58 mm form factor. They were used in the Signature (I), the Golfer and early versions of the Caddy as well - like yours - in the early versions of the MiniChamp.
They are notoriously hard to get, because the change to
retractable pens happened around 1999, and most stock has been dried out in the last 24 years...

I see two possibilities. When the pen is empty, use the grey plastic thingie with one of the newer,
retractable pens. Some tinkering is needed here, but it might work...

Or you or a member with 3D printer could create a new plastic thingie for use with the newer pens. Even different colors are possible. Good luck!
