What is this?

That's the normal air horn for the BBS, the stud clips into the two ears on the side.
Are you sure that vacuum port's threaded? All the ones I've ever had were just pressed lightly in.
And just to throw my 2 cents in, the BBS carb is the best carb for the slant six (stockish anyways), I have always gotten more performance and economy out of them over the Holleys and they are very easy to work on.