What is this open cavity in my 318 heads?

Syleng1 said

So it is also a special place that catches every small bolt/nut and “Jesus clips” that falls out of your hands while working on a carb.
Syleng1

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Yup! Dropped a couple of both in there recently.

And just for the record: You can't use a magnet to get anything out of there, either.

Or at least none that I have.