Putting the Mills pin (that split pin) back in can be a real chore.

     Try this.  Get a 4.5 inch x 1/4x20 cap screw.  (Standard bolt type
     thing with a flat hexagonal head)

     From the opposite side of the hole in the shaft where you wish to put
     the Mills pin, insert the bolt leaving plenty of bolt to pull out by
     the head.  This keeps the alignment as you tap in the mills pin.

     Now pull the bolt out until it's barely below flush at the hole on the
     armrest piece.  Insert the Mills pin with the chamfered end first and
     using a drift punch or another 4.5in 1/4x20 bolt head first, gently
     tap the mills pin into the hole.

     It will displace the other keeper/leader bolt as it travels through
     the hole.  As it's almost reaching a seated-in-the-other-side state
     make sure the keeper bolt doesn't fall out prematurely.  A couple of
     light taps and it's reseated.  You can use the bolt to drive it out
     again should you need to.

     Worked for me.

     al