Size unsuitable plug for 0.5 liter Nalgene water bottles

It was not specified (although the cap is proposed when buying the Nalgene water bottle 0,5 l) this cap could be mounted on the Nalgene water bottles of 0.5 l. In fact it goes as Nalgene water bottles 1 liter. It should specify the size, diameter of these plugs to ensure they properly fit the bottle gourds.