What piece is missing from this Holley carb?

I bought this 4 brl Holley at a swap meet. I have built several carbs, but this is my first Holley. Can someone tell me what is suppose to go in this threaded hole between the body and bowl?

Also, I found these numbers on the carb.

Air horn:

LIST 1850-1
1458

At the bottom of the threaded hole, I found these numbers: 34R 1585

Can any experts tell me if the missing part is easy to come by? Is it possibly a vacuum port that just needs to be plugged?

I haven't cracked anything open on it yet, but it is very clean overall. It has a manual choke cable bracket, but the choke butterfly and shaft are missing. Makes me wonder if it was used in a 2 x 4 setup.

For $2, I took a gamble.

 

what you have is a 600cfm manual choke holley the hole on the metering block is a vacuum port you can just plug it

 

May be a blind hole, Ive had a few like that, threaded but not drilled thru.... pull it apart and spray some carb spray thru it, if it goes thru then just plug it or put a vacuum fitting on it...

 

Yeah, I can see a hole coming from the side at the bottom of it.

I'll find a plug then.

Thanks!