Paul showed me his new masters for Sherman tanks he is working on now.
This engine insert is designed to allow drop in engine decks so different engine versions can be made and to ease the production of the resin models. Details like engine decks wear out faster than smooth parts.
This is the engine insert in place.
The entire kit will be resin. When this master is done, master molds will be made to make production masters. Those resin production masters will make the production molds.