It's true the weight and shape of the bob will have an effect on the air friction applied to the system, but for a pendulum of this scale, the effect of adding

It's true the weight and shape of the bob will have an effect on the air friction applied to the system, but for a pendulum of this scale, the effect of adding a coin or two will be a negligible change in drag compared to the relatively significant change in the pendulum's center of mass.