Hmmm, maybe some kind of Vasari alphabet?
Pros:
- approximately 26 "letters" (if you count them as shown in the opening post)
- none of these "letters" look the same (they look similar, but not the same)
Cons:
- not provable if no further texts are found
- difficult to separate the "letters" from each other
- totally guess-based thesis
Just a guess...