All I could find was this Premier Guitar article
http://www.premierguitar.com/Magazine/Issue/2007/Jul/Floating_Tremolos.aspx which states, "The original Strat tremolo was designed to float, meaning that the tremolo's rear edge is raised up off the body. Anyone who doubts that this is true should look at the patent drawings submitted by Leo Fender. That doesn't necessarily mean that you need to set your bridge to float - set it how you like it." I still can't say if that's a definitive answer, because I'm not looking at the patent drawings, but I still think it makes sense with regard to the nature of vibrato itself, being the wavering above and below the central pitch.
I wouldn't be at all surprised if the concept of locking it down came about later because of the string breaking/tuning problem or the double stop issues that have troubled Strat players.
Cheers........................................ wahwah