This is an advertisement for Jantzen swimsuits which ran in British Vogue in 1935.
Above is the 'Bra Zip' a smart model in new fancy knit with the uppers and trunks in pleasing and contrasting colors. The fastener permits precise adjustment of the brassiere with half skirt in front or skirtless.
Isn't it fabulous? I've seen the Jantzen 1933 'Topper' men's suit that has the top that zips off from the trunks. That was invented for decorum purposes but by 1935 the attitudes about swimwear had changed and most public beaches allowed men without to tan or sun sans tops. I wonder if the woman's swimsuit could have been the companion piece to the 'Topper'? What do you think?