Watching Gidget 1x03, The Great Kahuna. Noticed the big white square on the back of her bikini. Looks like a pocket. Did they used to make bikinis with pockets?
I assume it was for condoms.
A patch pocket.
Fashion is stupid sometimes.
She forgot to take the tag off? 
I can recall owning boy’s swim trunks from that era. Contrasting color patch pockets were standard.
Interestingly … boy’s / men’s trunks of that era were not much longer than her outfit. Far from the knee-length “board shorts” of the modern era. Trunks then were less form-fitting that her bikini bottom though. Probably a good thing.
We can’t see the front of Gidget’s outfit in the OP’s pic. But I was thinking you could probably make 5, maybe 10, modern bikini bottoms from the material in her one bottom. The top is maybe more like 3 or 5 modern ones from her single top.