I vote for 60s, and I'm going to digress slightly and say early to early/mid. I had an R & K with almost that exact shape/length, and it was mid 60s.... Also agree with Jenn that it's likely a blend, and I'm guessing linen/rayon or similar. The rayon would soften up the linen so it wouldn't wrinkle as much and would drape better.
That would be even cuter with a colorful scarf tied 'round the waist as a sash, or a chunky belt.