Jimmy, the person to ask on this might be Harley Cluxton. Although those Mirages predated his involvement in that effort, he is familiar with these cars and would know the answer. He could also ask John Horsman who would (obviously) know. You could also try to email John through the David Bull publishing site, and they might be willing to pass it on.
FWIW, my guess is that they had external throwout systems. Internal HRB systems weren't very reliable then, not even as good as they are now. Then again, race cars have all this stuff changed every race, even if it doesn't break, so the servicing issues that you would have on a street car wouldn't be a problem for a race vehicle.
Let us know what you find out.