1/2 inch BSP brass automatic bottle air vent (AAV) with shut off cap that can be used to stop auto venting if so required. The valve in the picture shows a brass shut off cap, sometimes they have a plastic cap (makes no difference to operation of the valve)
Please note that in certain circumstances air can be drawn into a heating system through an auto valve when the pump is running. If this is the case use the small cap to close off after venting has finished.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.