The Mckune Design Kyosho RB6.6 4.0mm Front Shock Tower is a direct replacement for the stock shock tower. The Mckune tower is precision machined from thicker 4mm thick quasi-isotropic carbon fiber material and weighs 7 grams. The tower improves durability when compared to the stock part.
Additionally, the +2mm is in reference to the amount of extra up travel the shock tower provides. It's recommended to use a longer shock ball link end to maintain the correct droop amount based on track and driver needs.