To my knowledge, Koufax didn't throw a knuckleball. What that likely is is his curveball grip. It looks like he has his middle finger on the seam, ready to pull down on the seam so as to add topspin to the ball. His index finger is up out of the way since it isn't used to throw a curveball.
The oddness of his grip (combined with a near-vertical forearm at the release point) could explain the remarkable effectiveness of his curveball. It may have helped him put more spin on the ball.
Thanks for posting the pic.
If you are really interested in Koufax's motion, I have posted to my web site a draft of a frame-by-frame breakdown of Koufax's motion...
http://www.chrisoleary.com/documents/PitcherAnalysis_SandyKoufax.pdfI view Koufax as something of an enigma.
On the one hand, his motion looks really good if you break it down. It's very clean and compact.
On the other hand, Koufax had horrible control problems for the first half of his career and ended up retiring due to elbow problems. However, Koufax fixed his control problems by not trying to throw to hard and I suspect that his elbow problems resulted from 1) combination of overuse and 2A) congenital circulatory problem and 2B) the use of steroids and other treatments that compromised his body's ability to heal.
All in all, I think Koufax has a lot to teach us about how to pitch.