Vintage 1981 Microwave Safe Campbell's Soup mug by Corning genuine porcelain

Microwave Safe Campbell's Soup mug Vintage 1981 Genuine Porcelain Corning Glass Works Made in Japan There is a dark colored speck smaller than the head of a pin, right inside the rim, about 1/4" down. This can not be cleaned off; it appears to be a small flaw in the glass. This flaw is visible in the picture of the inside of the mug; look between 10:00 and 11:00. The pictures are part of my description. Please look carefully at the photos and ask questions as needed. Thanks!