That’s commonly called a “Field control”, which is a slang term derived from the main manufacturer of such devices. It’s weighted, and used to control the over fire draft of fossil fuel burning appliances. It maintains an even draft, without regard to how many appliances are operating or how hard the wind is blowing outside.
If it’s a barometric draft damper I don’t see the weight or it’s adjustment.
**7M Field Draft Control **
For oil or coal fired residential heating applications. Designed for settings from 0.01" to 0.1". Features an infinitely variable screw adjustment. Employs side wings to control air direction with gate curvatures designed to compensate for differences inhorizontal and vertical settings. This 7M Field Draft Control is one of many top quality items in our Stove Pipe Fittings - Black department.
That would make sense in your case, since you had a power venter on the end. Probably needed that to keep from sucking some crazy high over fire draft at the furnace. That Field control probably tilts open a wee bit when the drafter motor turns on.