"Ricardo's Law of Association" by Jerry Emanuelson appeared in LC #13, about the summer of 1970. It was an algebraic proof that if two people work at different tasks at different rates, it is in their interests to specialize and exchange, even in one of them does both tasks better. While this was known to academic economists, it was not commonly accepted even by libertarians. Autarky (producing all of your own goods yourself), then as now, had strong emotional appeal to survivalists and retreatists.
Unless someone here has an archive of the Libertarian Connection, Emanuelson's original work remains lost. Neither Jerry nor I can find our copies. When I discarded my LCs, I kept his essay, intending to typeset it using Donald Knuth's TeX/LaTeX, but never got around to that. (I had it with another work, on the theory of elastic stability, partially completed.) Jerry told me that he has looked for his several times, but cannot find the right box to open. However, I did write to current owner of The Connection, Erwin S. "Filthy Pierre" Strauss, at his mail drop, but it will be some time before we hear back.
I wrote an introduction to Jerry Emanuelson's proof for my blog, here: http://necessaryfacts.blogspot.com/2017/07/jerry-emanuelsons-algebraic-proof-of.html
Jerry Emanuelson is very active in popular science, personal health, and life extension among other pursuits. His blog is here: http://www.futurescience.com/