If the pendulum bar has non-zero mass increasing the mass of the bob while keeping its centre of mass at the same location on the pendulum will move the centre of mass of the combination of the two parts down and increase the period. This is a small effect because the pendulum rod is very much lighter than the bob.

The pendulum in the clock colloquially called Big Ben is regulated by adding or subtracting from a stack British pre-decimalisation one penny coins. However in this case they are placed at the top of the bob not at its centre of mass so adding a penny shortens the pendulum by moving the centre of mass of the bob up