Originally Posted by Picchio Special

Good point - there were a lot of versions of this derailleur from 1972 on. Initially, they shifted better than NR, and could shift a wider range of gears, but perhaps weren't vastly superior. But the later versions, circa 1983-84 or so, used a drop parallelogram and were some of the best friction shifting derailleurs ever made. This is the version I was referring to re: the OP's bike, and the one that would have been extant when the OP originally built up his bike in 1983, give or take. The later Simplex SLJ derailleurs remain sought after for period-type builds when performance remains a primary concern, i.e. many classic-style randonneur and touring bikes. A case in point:

http://www.mariposabicycles.com/650b-touring.html