Hi Neil,
the kidney belt was to stop the kidneys banging around, so really its like a big old girdle to hold everything in its place. Mine had two velcro fastenes so you could get it nice and tight. It sort of sat just over my hips and under my rib cage.Id wear it over my tee shirt and under the jersey I wore when I rode. It was a big + for avoiding sore kids.... I was lucky... never passed blood, but it was the best investment, imho, after regular safety gear like lld gloves n' boots. No kevlar back then and the pants had some padding and there were armoured shoulder pads and vests, but I wasnt serious enough, or maybe a bit more foolhardy back then.