Fenton Header Holes?

So I just got some Fenton Headers for the 54 chevy and I'm wondering what the threaded hole on the bottom of each header is for? I was going to just plug them up with a bolt but they have got to be there for something, right? thanks!!

 

you'll be routing heat from the headers to the intake/carb with fabricated tubing. Inliners.org is a good place for all chevy six questions and stuff like that.

 

Oxygen sensor port if it's for an engine still made post-computer.