Yes, you're absolutely right. The BC54x transistors use the European pinout, which is the mirror image of the American pinout that the MPSAxx transistors have. There's also a Japanese pinout, and a fourth pinout that's used for some transistors that operate at high frequecies (radio, etc).
That transistor is used to stabilise the operating current of the output stage and it needs to be in good contact with the heatsink. I'm surprised that it's not clamped to the heatsink; that's how it's normally done. Here's how it works.
The output transistors in an amplifier operate with a steady current flowing through them. This is called the quiescent or idle current. As the output transistors warm up, their characteristics change slightly, and if the driving circuitry kept the drive conditions the same, the output transistors would conduct more current. This would make them get hotter, which would make them conduct more current, and so on, until eventually they fail, and then the fuses blow. This is called thermal runaway.
That little transistor monitors the temperature of the output transistors, and when they get warmer, it gets warmer too, so its characteristics change slightly too, and this slightly reduces the bias voltage to the output transistors. This compensates for the changing characteristics of the output transistors, and keeps the quiescent current constant. This prevents thermal runaway.
I'm a bit concerned that this amplifier was apparently designed with those transistors just standing next to the heatsink. This will delay the response to temperature changes, which I don't think is a good thing, although the forced fan cooling will probably have some bearing on this. I think we should attach the transistors to the heatsink, and once it's up and running, do some testing to see how the quiescent current varies with temperature.
I'm looking into the protection circuit and I'll get back to you with some more tests.
