Welcome to permies, Jimmy!
As Miles points out, nothing there to attach to a stack... plus, seems like a bit of a weird design if there is supposed to be an additional piece above that vent to connect to a stovepipe.
I wasn't sure if all inserts are meant to attach like that, so I looked into it.
Seems that it was common practice at some point to just stuff an insert into the fireplace opening and let it vent into the fireplace, trusting the fireplace chimney to do the venting.
It also seems like this is a real bad idea. This (charmingly named!) chimney sweep site explains it pretty well.
http://www.yourashismine.com/id27.html
So, rather than a missing part, it could just be a bad idea sort of insert... Either way, unless a good solution for attaching to a proper stovepipe turns up, I'd be passing on this one.