Help identify a mystery brass part along the water line
As the "Spirovent" label should lead you to believe, yes, it's an automatic vent (albeit one that looks rather gunked up at the vent.)

This would appear to be the instructions for your device:http://www.spirotherm.com/sites/default/files/JR%20IOM-B.pdf

You should also look for manual vents along the loop at high points. This helps keep pockets from forming if working correctly, but does not remove the need for manual venting if the system was drained and refilled, had a leak, or otherwise has lots of air in it.