The mystery buyer who dropped $3.7 MILLION on a rare Honus Wagner card is revealing himself ... and it's real estate mogul Kurt Rappaport!
The card is the legendary 1909-11 T206, which Rappaport describes as "the HOLY GRAIL -- the rarest and most desired of all sports cards."
The purchase -- handled by Heritage Auctions -- makes Rap's Wagner card the 2nd most expensive baseball card ever purchased ... behind a rare Mike Trout card that sold for $3.9 million back in August.