Not sure if this has been posted before, but I just came across this very useful looking document - complete with colour pics and description of the evolution of SA trigger shifters from 1938 through to 1960:
http://genetics.mgh.harvard.edu/
I can't speak to the accuracy of the information provided, however, as I am not an expert on such issues.
Paul Williams, Ottawa, ON, Canada
Paul B. Williams, BAH, MPhil, PhD
Ottawa, Ontario, Canada
castell5@sympatico.ca