So far I have found only one brave attempt to interpret the Pictish symbols. This is immediately a rather uncommon interpretation though. The website is called “Pictish Mithraism” which also tells you in what direction the interpretations go. The author argues against several of the standing hypothesis, both the symbolic and the historic ones. The biggest merit of the website is that the author categorises the symbols and tells his readers how he came to his interpretations.
I have the idea that I knew that Penny was working on turning his website into a book, but I am not sure. When in Scotland in September 2018 I ran into the little book. What I wrote about the website also goes for the book. It is still the only attempt that I know to explain the symbols and the author does that by grouping the symbols. Penny helps even those who do not agree with his hypothesis. I must say that after these years, I still find the hypothesis daring and interesting, but little convincing. The book does add something to the hypothesis that I do not remember from the website though.