Newb with a Newb question

Can someone tell me what this thing is ?!? It’s above an older Weil-McLain boiler. Is it a pressure reducing valve (assuming no) an old school expansion tank? It looks like it’s leaking I’ve never seen one before

Just what it says, air scoop, removes air from the forced hot water system, air then vents out the top. Rust indicates water release, vent may need adjustment so only air is vented.

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