The Sheaffer K-style ballpoint refill in medium point, black ink. This is the standard fitting across most of the Sheaffer ballpoint range, including the 100, Sentinel, Icon, Reminder and Prelude - the notable exceptions being the Sagaris and Taranis, which take the T-style refill instead.
Genuine Sheaffer, with the specially formulated ink the pens are tuned for. Worth keeping a spare in the drawer.
