I never heard of this couple before. Having looked at the pictures, the comments and the links, I have a slightly different take of this couple.
I believe they were foreigners. Their resemblance to some of the Argentines is interesting in that most people in Argentina are an ethnic mix of Spanish and Italian blood. Argentine women, especialy of the upper classes, are very beauty conscious. I do not believe the couple is from Argentina, however, for two reasons. One, if they had just fled from a brutal military regime in their home country, I doubt that they would be driving or hitchhiking around the United States. Two, while Argentine customs may have changed over the years I cannot imagine an Argentine woman not shaving her legs.
I do not believe they are Italian nationals either. If the initials on the watch are in fact the man's, the letter J is not customarily used in the Italian language. John is Gianni. Joseph is Giuseppe. The month of June is Giungno.
The olive colored complexion certainly implies the Mediterranean area which would exclude their being Northern, Central or Eastern Europeans. I doubt based on their complexions that they were French Canadians.
This, in my book, leaves us with Spaniards, Portuguese or Mediterreanan French. I do not know what the customs of women were back then, but I suspect French women were the group least likely of the three to have shaved their legs back then. (the man was clean shaven so it is not that they did not have any razors at their disposal). The French ancestry would include former colonialists in North Africa.
So the question still remains how did this couple (probably French) of comfortable background get to the US, meander about and meet a violent death without anyone back home worrying about them?