In 1937, Warner introduced its Alphabet Bra with four cup sizes (A, B, C, and D) to its product descriptions. Before long, these cup sizes got nicknames: egg cup, tea cup, coffee cup, and challenge cup, respectively. Two other companies, Model and Fay-Miss (renamed in 1935 as the Bali Brassiere Company), followed, offering A, B, C, and D cup sizes in the late 1930s. Catalog companies continued to use the designations Small, Medium, and Large through the 1940s. Britain did not adopt the American cups in 1933, and resisted using cup sizes for its products until 1948. The Sears Company finally applied cup sizes to bras in its catalog in the 1950s.

Can we conclude that is the letter the cup size in a bra?
yes