No, it is not. The PR 99 was a pro version of the B 77, and the Sonorus coincidentally comes in at more than double the amount of the re-released B 77.That is not a 77B III
Still the bones of a B 77. Frame, motors, heads.
Sonorus calling it an ATR is an insult to the Ampex ATR line, can't tell if intentionally so.
But there is certainly more to the tape path than just throwing another roller, or roller guides in. But that's another subject.
My point is simply this, one can pick up a nice example of a B 77 or a PR 99 for that matter - for under 1500.- and pay a tech another, say 800.- or so, to service it and to bring it up to spec. Just takes time and effort to hunt a nice one down.