The key words here are "at the time." What was done "at the time" was part of a collaborative effort to create THE WIZARD OF OZ. Guys, I only chimed in here because I think our cultural heritage shouldn't be tampered with (or tampered with as little as possible -- transferring movies shot on...
I'm implying that I couldn't possibly know what the intentions of the filmmakers were in 1939... and that I personally dislike having 1939 works of art made to look like they've been made in the 21st century.
I remain unconvinced that the wires would not have shown in 1939... I could ask my mother (the only person I know who is still alive and saw the movie on its original release... but I can hear her sarcastic reply... "Oh don't be ridiculous... who would care?").
But whether they showed or not...