I am old enough to have worked on cars from the 1930s and earlier. I never heard of black pepper in a copper radiator, but some, when they had a small leak, would put uncooked oatmeal into the radiator, it would travel to the leak and clog it for a period of time. A number of things need to be kept in mind with the old cars: Radiators were copper, not plastic, people ran water in the radiators or, in the Winter, one season glycol (or alcohol). Some water pumps had externat lubricating systems and didn’t need radiator additives, engine block cooling passages were much simpler, as was the heater core. I would not recommend oatmeal in any car past the 1940s. Oldmotorist