What I see in print (contents page) is 'forward slash letter forward slash' /r/. The contents page is the only place in the text where these slashes around a letter appear.
On the print pages (2-3, 22-23, 42-43), the text shows 'bold italic r' r, talks about syllables and words that contain r, and the sound of the r in those syllables and words. Nowhere in the text is there a stessed or unstressed diactitic. Neither is there a phoentic display of a word.
So, to directly answer your question ... I see slashes around a letter, words that describe the strength of that letters sound in a word, and this, along with my limited experience with phoentics, leads me to belive that it is a phonetic enclosure.
I posted here because the error warning light in my minds eye is in overdrive over this slash and my assumptions about what it is and how to present it.