So it looks like speedmaster uses either of the cheap castings. Those are the same as the sidewinder, this is the other cheap casting

http://www.ebay.com/itm/Mopar-Chrysler-BB-383-440-265cc-74cc-As-Cast-Bare-Aluminum-Cylinder-Head-/181870711810?hash=item2a5856d402:g:vmQAAOSwT6pV1ShD&vxp=mtr

Even easier way to tell em apart is the heart shaped chamber in the pro-comp or as it is labeled in this pic a "speedmaster".

I am not causing global warming, I am just trying to hold off a impending Ice Age!