I don’t know what you mean by junction box.
It is the bendy bit that joins the two header pipes to the exhaust can. It forms a junction between the exhaust can and the two headers, hence my calling it the junction box. Officially it is named as the “Central exhaust pipe part number 977664” on the Fowler’s parts website.
https://www.fowlersparts.co.uk/parts/view/977664