To be unquestioned 'Fully Split Bands', the top of the bands must appear as the top of the old style loaf of bread.
Split bands can be the the result of a good strike off a die that is good enough, and the coin can still be less than genuine MS60, (coin very slightly worn)
or MS60 and / or better, the dies still be good enough to produce split bands, but sufficiently worn to produce flat band tops. (die slightly worn)
Both die and coin need to be in top condition.
That is why I like the abbreviation 'FSB', rather than 'FB' or 'SB'.
That is why 'FSB's are so scarce.
It is also highly desirable the have the obverse fully struck up as well, which are more frequent on earlier dates in the Mercury series.