That's the factory supplied "Jesus" clip catching spot. Also handy for catching anything small enough to drop in, and important enough that you cant run without it. BTW, the intake block off gasket may not seal that hole. I dont thing it's much different fromt eh std bathtub...it just doesnt have the stap removed for the crossover. I think that area was left open, to help stop the heads from cracking there from too much heat. Or perhaps to drop the crossover temp slightly. In any case, it being open, means nothing...lol.
