It was about 67-68, if memory serves, when CBS...yup, THAT CBS...bought Fender and immediately started mucking about. Many players of today think that the larger headstock was made to give the guitar greater sustain. And a great number of 70s and later players actually prefer it. But the sole reason for the big headstock was they could put a bigger logo decal on the guitar. Pure marketing. Perhaps that's a 68...could very well be.
EDIT...just checked...like Spacey said it was actually 65/66 when the change was made.