The illustrated inscription was written from right to left, so it was a backwards letter. According to the caption, it was the letter D. It would usually have been written just as it is in Latin and English, and I'm not sure why it has a tail here. P and R in Oscan were written as they are in Greek (Π, Ρ) except mirrored.

Cyrillic Я is a variant of Old Cyrillic Yys (Ѧ).