There are certain characteristics in this signature that I've never seen in a psa, jsa or steiner Koufax. I have seen them in Koufaxes authenticated by Christopher Morales, MFP Reborn, etc. that being said, I'm gonna say likely not authentic
the pathetic attempt with the "tail up" S from the start is a tell tale sign of a forgery. The Tail is the Tale Tell. I like the way that rolls out. It's not close to a real Koufax, it's as fake as hell. I hope that clears it up for you.