1977 on RHD dual brake master with integral servo:
Next three images from John Maguire in Australia:
From 'How stuff works' but I believe the drawing to be incorrect, the passages from the reservoir to the right-hand cylinder are different to those on the left.
I.e. the inlet or equalisation port should be connected to the space between the two seals on the primary piston, just as it is on the secondary. The locking pin is also omitted above, this is necessary to position the secondary piston in exactly the right place between the two ports with the pedal released. Also it seems logical that the secondary spring should either be stronger or compressed more than the primary spring with the pedal released, to be certain that the primary spring isn't pushing the secondary piston forwards off the locking pin.
The push-rod from the servo acts directly on the master cylinder piston: