[ableal]

Such a stylus was featured in a short story set in Cicero's time (he even shows up at the end).

[spoilers]

It's "Mightier than the Sword", by John Maddox Roberts, a historical mystery tale. The triangular shape of the criminal's stylus is key, and he's caught when the investigator asks to borrow it:

"Actually, I didn't really forget my own stylus today." I took it out. "You see, the common styli are round or quadrangular. Mine, for instance, is slightly oval in cross-section." Cicero and his friends drew out their own implements and showed them. All were as I had described. Cicero's was made of ivory, with a silver scraper.

(Part of the writer's SPQR series - I find them well researched and not leaking too much of the modern mindset into classical times.)