Ah, now I understand! There wasn't one in my rear drum and I thought, looking at the diagram that it went between the dummy axle and the inner (bearing) side of the drum. But now I see that it rests one step further out on the dummy axle and presses on the brake plate.
Well, nothing seems to rub, so I guess I don't need one!
But just for reference, are these 0.015" in thickness?