Sierra estate rear arms. not the one you have







Looks like someone may have cut off the shocker bracket on the one you have. Guess it was on a kit car?

There are a few rear arms for the sierra if yours is a disc braked type. one for none cosworth i.e xr4x4, gls4x4, ghia4x4,dohc 4x4.
You can tell by the shocker mount is this shape



The cosworth ones are like this




Just be aware that there are different toe and camber setting between the arms. If you dont have a match pair you will find the car driving very odd.