wasn't that pad originally intended for a tube to connect to an old style choke mechanism ?
if i'm remembering right, some old carbs used the crossover port to pass exhaust into and out of the choke.
some tubes didn't get direct exhaust passage, but the holes were close to breaking through the casting, which allowed the quick transfer of heat to the tube, which was generally 1/4" od copper.
if i have this wrong, could someone please correct me ? or at least clarify what i'm trying to say.