I think the explosion of interest took place in the early 60s when the harmonica was used in pop music. Beatles "Love me do" had harmonica on it, and the blues boom spawned Rolling Stones, Manfred Mann, Yardbirds and their various copyists, all of whom featured harmonica. Sonny Terry earlier played harmonica and there were many copyists of his style around the folk and blues clubs of that period. I think it was then that Hohner brought out the Echo Super Vamper which was the 'harp' of choice for all the blues players. I bought mine in several different keys from Bells of Surbiton, who were Hohner main dealers. I'm pretty sure they were available in every key, and I know several players who seemed to have a full set, which was necessary as guitar players were using capos and playing in all keys. Bob Dylan was also a big influence in this - we all had harnesses made of old wire coat hangers!!