Well it looks 70s to me and I have had many comercial items from the 70s with metal zippers. probably early 70s.
I would "think" Mexican would be an appropiate term, but I am not the best with terms. I am still leraning. Very cute and should sell easily. It was probably yellowed because it is cotton and thats typical, and a good clean freshened her right up.
I think 70s but not too sure on the term. Very Hippie ethnic, one with nature like to me.... so not too sure on the mexican term maybe boho? Help here all!
Also as fat as metalzips, which has been discussed. In 1968 the nylon zipper was adopted commercially, though they had been around for DECADES. There were some manufatures who didnt transition immediately and others who still used meatl zippers for certain styles etc etc. I have modern dresses with metal zippers (high end dresses)
So plastic zips vs nylon zips are good indicators when Combined with other factors such as sizing, label and silhouette.
Hope that helps.