I thought I would be able to give you an answer. I collect Buffalo's and have a 1915 PF in an NGC
holder myself that I recently purchased. I also consulted with Lange's Complete Guide to Buffalo nickels
and Bowers' Guide Book to Buffalo's and Jefferson nickels
. My first thought looking at the image of your coin was that it is definitely a Matte Proof based on the granularity and overall look. however, Lange says that that is deceptive, particularly for the 1915 issue which in better circulation strikes can look a lot like the Matte proofs. Next, I noticed that your coin does not have a flat rim all around on both sides. A proof should have this, according to Bowers. But I looked at my 1915 Proof and it looks not quite flat at the top either.
Moreover, Lange says a diagnostic of the Matte Buffalo proofs is a small triangular defect to the right of the "E" in UNITED. Your coin does appear to have such a defect. Now that I have thoroughly confused you, I would say that you have a 1915 Matte Proof, toned, that allowed PCGS
(correctly or not) to avoid answering the question.
I attach images of my 1915 Proof:Image: 272164_o.jpg
38.21 KBImage: 272164_r.jpg