In the 1940s corset manufacturers saw a gap in the undergarments market. Corsetry was losing ground, but the new more revealing swimsuits really needed experts to design garments that hid faults in a woman’s shape. Manufacturers achieved this by adding stretch tummy control panels to hold in the stomach. Most also used bra cups and boning to give bust support.
Fashion swimming costumes could then be worn either strapless or with small straps that buttoned onto the inside or worn halter style as in this example. Even then women were not keen on lighter body marks that may have made a strapless dance dress look less attractive.
Women still continued to wear all in one swimwear in the 1950s, rarely wearing a daring two piece bikini.
Zips were still used in the centre back of swimsuits retaining the corset like appearance until the early 1960s. Sometimes zips were put in the side seam. The swim suits of the 50s and early 60s were cut straight across the top of the leg in the form of a modesty apron that hid the separate matching fabric crutch. Subtle changes occurred in a few years and the modesty apron style soon looked old fashioned.