Interesting. On following up on your links, I see that while optical pumping was invented first, Townes was able to make the first maser by using an electric field to separate out an excited state of ammonia, achieving a population inversion without pumping. http://home.fnal.gov/~kubik/FermilabWebsiteDocs/Masers.ppt

The closer you look, the less the history of innovation looks like a simple sequence.