Pentiment

The existence of several pentiments in the British Museum drawing, for example in the left foot of Christ, the right hand of John the Baptist, etc., not to mention the obvious general stylistic resemblance to the work of Andrea del Sarto, proclaim it as an […].

And one of the things that immediately appears is this thumb, a first position of the thumb. It's called a “pentiment”, it's an artist's first idea, the thumb was subsequently moved to a different position. And you don't usually find that in a copy, but it's not proof that it's definitely the original.