Greetings,
The 1/8" plastic piece is press fit into the ferrule on the end of the metal follower rod. Usually, you can push it into the part with little effort.
I will cut you a piece and mail it to you.
I trust you have the standard issue Star Primer Tube and not the extended 100 primer version?
When the plastic piece is inserted, it just falls into the primer slide with little excess to remind the operator to recharge with primers.
Cheers,
Dave