This is a simple pen shell for “magic” disappearing ink cartridges used for fabric marking.  These cartridges (used for sewing but also for my kids' magic ink workbooks) did not fit in any pens I had, so I made a custom shell. 

The refill cartridge I designed it for measures approximately:

128 mm tip to end

15 mm tip to plastic collar

1 mm thick plastic collar

3.2 mm diameter plastic shaft just below tip

5.5 mm diameter plastic collar

4 mm diameter main shaft