Who's Hand is This?

I was sorting through some 1968's tonight. I came across this Roberto Clemente Sporting News All Star card, number 374.

I flipped over the card to see what awesome things The Sporting News had to say about Roberto. No dice.

What I saw, was this; The left hand and hairy wrist of an unknown player, and bat knob.

Is this a puzzle piece, and baseball card all wrapped up in one? Ooh the intrigue!