What engine part is missing from my 04 210?

If it wasn't drilled out, it would look like just a piece of cast iron with no hole.

Yours looks drilled out and the little hole below it is the oil return to the oil pan.

Since you have a hole that will let oil leak out and you don't have a block off plate, you should get a block off plate like in the link below

That's just one example, they're easy to find and cheap and will stop that leak of yours 

An 04 Excalibur with that hole makes me wonder if your engine was rebuilt at some time before you got it, and an older style block was used and nobody put a plate there since the original 04 probably didn't have one (just an opening to nowhere, with neither hole drilled so GM skipped the plate)

When I was winterizing my 04 120 w/ the PCM Excalibur over the weekend I finally pulled out the hard tanks to get a better view of a small oil leak that has been occurring since I bought this boat a few years ago.    Upon inspection of the lower rear port side, I noticed this area that looks like it is missing a part based on the bolt pattern and what may have been a gasket at one point in time.   At idle, I noticed a small droplet of oil spitting out of the large hole.   As I give it more gas, the oil volume increases a little bit.    What goes here?

(The picture should be rotated 90 degrees clockwise but I'm not sure how to do that here)