What's this part?
 

and what, if anything, is missing in the hole?

http://forums.pelicanparts.com/uploa...1341934663.jpg

That is a lead plug blocking a fuel gallery, I can't tell from the pic for sure but it looks like it is there.

I had one pop out on me when cleaning up my webers, I just used a small fishing lead weight and a punch to push it in the hole and cleaned up the excess with a razor blade, 1000 miles later still not leaking!

Ah, ok I get it. It's missing, for sure. It's just a lead pellet that's stuffed in there? Can I use something else as a substitute?

A couple more pics, and you can see that they're there in the other 2, but not in this one:

http://forums.pelicanparts.com/uploa...1341968336.jpg
http://forums.pelicanparts.com/uploa...1341968464.jpg

I think you have a few options, but as I nderstand it the danger is installing something that sits to deep in e passage, leading to issues such as blocked passages. I know a few guys have described using a ball of silver solder if you have that available.