It looks like some kind of valve.

I think you are right Earle - I believe this type of valve was called a turncock since you could regulate the flow of whatever by turning it to change the size of the opening in the piece relative the the hole in the pipe or container it was in. The threads kept it from popping out of where it was placed. That one was likely part of a water faucet when it was in place.

Here is a link to a site that has a drawing of one that looks somewhat similar http://www.thefreedictionary.com/turncock