Early Parker 21 and 51 cap activated ball points have a brass mechanism (Parker called it a "ratchet frame assembly") that is prone to cracks in a critical sleeve that holds the mechanism together. When the sleeve cracks, the cap will eventually fall off with half the mechanism still screwed to the barrel. Unscrew the jewel (on a 51; sometimes easier said than done!!) then the clip screw to remove the mechanism. The smooth brass sleeve on the left is a force fit over the mechanism on the right. The brass sleeve is what splits. I've silver soldered a couple of these, but epoxy should work too. There should be a color difference on the right hand piece to tell you how far to push the sleeve - this is critical. The top of the mechanism is keyed to fit into the key for the clip.
Note the spring - it has to sit between the inner piece with the fingers and the sleeve and is compressed by the piece on the right. It will fall out when you aren't looking... If you find a pen missing the spring, you can make a replacement from a G-string.
The fingers on the left side have to be rotated 90 degrees from the indents on the right (it's really the only way it all goes together properly. Make sure the spring isn't binding before you glue/solder!