Well, son of a gun - I don't know how we missed that all these years!
Fortunately, as PB said it makes no meaningful difference in the operation of the amp.
I can see now how it came about - the diagram was first made before the prototype was built, with the 0.51-ohm dropping resistors in the power supply diagram. After it was laid out and built, those resistors got moved to the output stage diagram to better reflect the physical build, but the small change to the topology was not noticed.