The situation that requires a
Mercury dime to be Fully Split Bands is that usually requires
the coin to be in at least MS60 or better condition, and that it is struck from fresh dies. Even then, some Mercurys may not be FSB.
The band detail needs to be sharply struck up enough for all bands to be clearly separated along the whole of their length, without interruption, and 'Fully' means that the tops of the bands are of the shape that resembles the rounded top of a loaf of bread.
These requirements make FSB Mercurys rather scarce, some would say 'rare'.
You may be lucky enough to get only a few from hundreds of Mint State 60 condition or better examples.