I've taken some notes from a post in L&T archives on Mary's silver knife. I also thought of little Mary's silver knife in Mansfield Park when I read this letter. In JulieW's opinion, Mary's silver knife was used for eating fruit.

Reference: JulieW. Only the grandest would have used. Sept 8, 2005. www.pemberley.com/bin/archives/41582

This silver knife Jane suggested as a present for Mrs F.A is probably a folding knife used to eat fruit; a silver blade will not taint the delicate taste of fruit.
It was a genteel habit to ue a silver folding knife to eat fruit when not at table or between formal meals.
Jane may have thought a silver knife would be a suitable present for a lady abut to travel or on holdays.
Although Mary's visit in the Island was probably shortened by Frank's sudden return.