This is an Australian made stainless steel flue water jacket that is designed to replace the first section of 6 inch (150 mm) flue.
It is a 2.4 kW water jacket and it takes the heat as the smoke/gases leave the fire-box. Be aware that these gasses cool rapidly as they give up their heat to the water in the water jacket and as result the resin/creosote in the wood forms a layer on the inside of the flue water jacket meaning that the flue will need to be kept clean, lest the layers of resin form an insulating barrier to the heat reaching the water. See the drawing for dimensions. The ports are 20 mm ports