Modular Terrace Kit MTK2 Grimy Stone Back Yard Wall
Needed to complete.the Twin Back Yard: two for the first unit and one for every unit adjoining thereafter. Matching units all share the MTK2 GS1 prefix.
The screenshot shows all the parts for a row of four GS1 cottages in exploded form: the image is also available in larger size, along with an example of a completed kit, in post #20 in this forum thread: