Extra barrels. Everyone is offered the bulk pricing of $1 each, no matter how many you order. AT-style so they will work with most original Model F keyboards.
I also have available XT style original IBM barrels that will work for all Model F’s with the XT style layout – same cost; please specify in the notes if you’d like the XT otherwise you’ll get the new AT style ones.
Note: Ships with the F62/F77 orders. These do not ship free unless you have ordered a keyboard or key set.