I wonder if any of them are still usable. I would guess that new pens are NLA and whatever pens that might be around are long dried out.
Oil-based ballpoint inks tend to last a really long time, especially if sealed.
You might be surprised, I picked up a box of new/oldstock sealed in packaging pens for the HP 7475A, they still work just fine. Also I have some of the weird little ballpoint pens that are used for the Atari 1020, they also continue to work.
I have one. The pens work fine.