On USA made York Olympic plates, 45 and 35 pounds, you will see "swirl" marks on the back of the plates. This indicates they were milled down to within a very close tolerance to the announced weight of the plate itself.
Most 45s and 35 were cast slightly overweight and then ground down. If you pick up an Olympic plate, any manufacturer's, and there are no such marks on the back of the plate, it means it was not subjected to the milling process. In such cases, obviously, it is "cast-to-weight." What tolerance range an indivicual plate falls within in such cases can vary widely. To give you some idea of the extremes this can reach, until fairly recently, imported (China) Olympic 45-pound plates were not milled in order to keep the overall price down, and they commonly ranged from 38 to 50 pounds from plate to plate. Even when York Barbell started importing a cheap line of Olympic plates, this was the standard out of Chinese foundries.
At one time, York Barbell also milled their USA 25-pound Olympic plates. However, this stopped by the 1970s, if not before. Obviously, the smaller plates are more easily cast to within a reasonable tolerance, and, hence, do not require milling.
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