Both of those are step through frames - true mixtes have a top tube that goes all the way back past the seat tube meeting the seat stays at the rear drop out. I suspect Xeney is correct, there were some around before the 70's but, it seems that they became quite popular at that time period as an intermediate step between a heavy "women's" step through frame and a men's racing bike. They allowed a low top tube, for womanly propriety, but were stiffer, stronger and lighter than a traditional step through.



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