I thought it was more a case of Leo being the astute businessman and saving more money on the builds. Alder was cheaper than ash, and with solid bodied guitars, it didn't matter that the wood had minimal figuring in it, as it was all covered over. So from about '56, sunbursts, clears and blonds were normally ash (and usually cost a bit more as well), whilst solid colours were normally alder (which also did away with the need for grain filling). As always with Fender, there were exceptions.