This is the only explanation regarding the use of the stylus that I have ever been able to find!

The following is quoted from Mr. Mohan Daniel of the Serendib Gallery::

Stylus
"The stylus is a 'ULKATUVA' used to train a student to write on a palm leaf. Once he is trained he is permitted to use a different type of stylus the 'PANHINDA'."
The latter is shown below with an Ola leaf manuscript, courtesy of Serendib Gallery.

He also notes that a "student" was not necessarily a youth. In fact, this would not likely be the case as these knives were made for the Kandyan kings and worn by them or other chiefs. More on this under the "Mediaeval Sinhalese Art" link.