Found it. You have a 20SF.
Look at Hammer's 20 . . . old gallery. Transom thickness is not 2 sheets of 3/4". If I remember correctly the thickness of the core is 3/4" + 5/8". If you are raising the transom there are some issues. I know with the 20MA's if you raise the transom to 25" the bottom bolt holes put the bolts right through the splashwell floor . . . I'm not sure if that's the case with the 20sf, but something to consider/plan around.
http://www.classicseacraft.com/197120CC/index.html