The PCB had VI limiting for short term overload protection. A fuse in series with the speaker was used to disconnect for longer term low load protection.
Note the addition of diode D3. This makes the asymmetric output stage more symmetrical. This worthwhile add-on was often used in output Darlington pairs and was often referred to as Baxandall diode. This is the first time it was used in an output triple emitter follower,