Genuine Rover Chrome deluxe side body trim. Sold per side.
There are many cheap copies but this has the original extra lip to give the correct profile which makes it slightly easier to fit but looks correct.
Heat with a hair drier to help get it fit the bends on the bodywork (or carefully warm up slowly in oven to make supple).
Do not use hot water as it may cause discolouration. Originally BMC suggested a rivet at either end to stop moulding pealing off.
Fit from rear forwards. 3.353 metres long in case you need it for specialist jobs.