[pvg]

Does anyone know much about this author/site? It looks like some sort of Zoroastrian/Zoroastrianism-enthusiast and let's call it 'independent researcher'. There doesn't appear to be much in the way of external references to this voluminous work and some of it seems a little tendentious or at least, at odds with current consensus.

Sites like this are one of the joys of the internet but it's usually easier to figure out the angle.

[jagaerglad]

Can't say much about the linked page but one could read the article[0] about the same topic on Encyclopedia Iranica, which is a better source since it's compiled and edited by scholars in history, linguistics, iranian studies etc. Haven't read them myself but they both mention the epigraphist Grotenfrend at least

[0]: https://www.iranicaonline.org/articles/epigraphy-i

[retrac]

The Cambridge Encyclopedia of the World's Ancient Languages: https://www.cambridge.org/ca/universitypress/subjects/langua... (available elsewhere for less...) also has a good chapter on Old Persian.

The chapter on Avestan is also relevant and interesting.