Eliza,
I remembered something else that may be of interest. New Agents Class beginning on 2/4/80 was issued .38 cal. Model 10s with a 2 1/2" heavy barrel. By October of 1980 new agents were being issued .357 cal. Model 13s. This should help narrow the time frame of the transition to Model 13s. I liked them both but really liked the Model 10.
D