In my opinion, this is likely not authentic. Formation and slant issues and portions of the signature appear to be carefully drawn. You can see how slowly "antle" was constructed.
I agree with the others, but Mr. Zipper and Cee Gee know Mantle much better than I do. The slowness gives it away big-time, but the handwriting is clearly off to me, especially Mantle.
What's the story on this, JBS? Can you post a link to the auction?