Hello,

The blue ring rest inside the blue circle I made(In Picture). The ring had threading on it so you would be able to screw on the cap back on.
(I know I posted this in the Repair Q&A forum, If this post is violating anything please feel free to remove it.)

Any one by chance would have a spare?

this is a piston filler, isn't? I think I have the part you're looking for, in fact, I believe I have most of that pen.