Does this count as highly localized?
No, that isn't what I was thinking about. Yes, it is localized to that general area, but what I meant by highly localized was more like this, which is what I found on the TO-220s on the 18V circuit on my SR-2A (Q405, which corresponds to Q207 or Q210 in your TA-3A). I was able to scrape some of that off (might have been some glue or other compound applied at the factory, or just blistering?), but the brownish stain you see around the edges remained. In any case, this would be consistent with the heat from Q405 being carried down to the board by the legs, and the board damage being highly localized around the legs. There was barely noticeable staining on the underside of the board at these spots, and solders (or surrounding ones) were not ruined, as yours seem to be.
I don't have a TA-3A so I can't compare directly, but my two TA-2As do not have what you show on the boards or even more local staining, and I continue to be a bit perplexed as to why your nearly black stain extends so far and so evenly from the devices. Perhaps someone with more repair experience can shed some light on this.
In the TA units, as you know, there is an logic board overlying the area of the 18V circuit, which does not help heat dissipation, but still, it seems odd. Looking at the schematics, I also note that in the TA-3A circuit there are 4 TO-220 devices, whereas in the TA-2A there are 2 TO-220 and 2 smaller, lower-power TO-92 or related devices in the equivalent positions. So there probably is more heat being dissipated by this circuit in the TA-3A, but I am reticent to push the analogies given the differences (perhaps I've gone too far already).
Nonetheless, I don't think what you are observing is normal (I think the Nak engineers of that era were better than that). That is why I was wondering if there might have been an episode of overheating, such as the air vents on the top cover being temporarily covered at some point by some papers, etc., or some other event that caused that. Just a thought.