A pin-up girl or pin-up model is a model whose mass produced pictures see wide appeal as pop culture. The term "cheesecake" is synonymous with pin-up photo.
The earliest documented print usage of this sense of cheesecake is in 1934, predating pin-up. Probably used originally in the phrase (said of a pretty woman) "better than cheesecake.
LINK: All photographs are from LIFE MAGAZINE and dated May 5, 1950. Photographed by Edward Clark.