【 コーヒー牛乳 (kōhīgyūnyū) 】coffee milk
銭湯 (sento) public bathhouses became popular around 昭和30年 (Showa 30's) 1955.
Refrigerators were also becoming popular around the same time, but like bathtubs, they were not yet common in average households. Refrigerators, TVs, and washing machines were still a luxury.
Therefore, going to the sento for a hot bath and buying a cold drink afterward, was quite a treat.
So what is the association between a 銭湯 and コーヒー牛乳 ?