There should be one ... I created the prototype when I had not all techs researched, so at the time of creation, it was a Future Prototype. Now that I can create it with my current tech, if I read correctly what Steve says, there should be in the ship design window, upon selecting it, as per
"If you select a prototype component on the Class Design window that can be created with current technology (i.e. not a future prototype), you will see a 'Research Proto' button appear. Clicking this turns the prototype into a Research Prototype. Research Prototypes have an (RP) suffix on the class window."