We broke the story ... Mantle scribbled the notes on a PGA scorecard right before he gave his last public speech (which was nationally televised from Baylor University).
One of the most memorable lines from the speech -- "I didn't know how Lou Gehrig could here [sic] at home plate knowing he was going to die and say he was the luckiest man on the face of Earth [sic]. Now I think I know."
The item was part of a big sports memorabilia auction put on by Grey Flannel Auctions -- which also featured a Michael Jordan game-worn rookie jersey ... which sold for $25,076.
There's more ... a 1927 New York Yankees "Murderers' Row" signed photograph -- with autographs from Babe Ruth and Lou Gehrig -- raked in almost $72k.
So far, the identities of the buyers are unknown -- but clearly, huge sports fans.