In short, if I had to guess I would say tempered 7075 (GUESS) given the application, fatigue requirements, appearance and industry. But I also recognize that even with the part in my hand I'd be HARD pressed to give you an accurate answer without the proper testing. (And even then it would be ridiculously expensive to get much more than the first digit of the series)
Oh, and I probably should have said this at the outset, but call the manufacturer and ask. It's the type of situation where they want you to know that they know, you know?