Alex:
It seems to have taken Delta single-control faucets a long time to "take off" - those mid-1950s ads I've seen are little ads in the back of Better Homes & Gardens and American Home, among other publications. I'm sure you've seen this kind of ad in vintage mags, for all sorts of specialty products for the home. From the modest ad size, I'm assuming that they started small, and word got around about the convenience of the faucets from there, turning them into a bigger company.
The Delta Faucet Website gives 1954 as the date for the introduction of their single-handle faucet. The 1950s ads I've seen show them looking identical to today's, though I'm sure there were differences that aren't visible.
I know single-handle Deltas are certainly my own preference - except when it comes time to rebuild one with new O-rings, LOL. [this post was last edited: 9/22/2013-18:45]