This is a pair of outhouses for the Twin Terrace Unit. It's double ended, to give more variety, and again it's designed to 'slide inside' the back of the terrace. It's also revolvable to allow for yet more variation: after all, the backs of terraces, seen from railway lines in almost every British industrial city apart from those in Scotland seem to present a bewlidering variety of additions even within the same row....
The 'exploded version' to the right of the screenshot should make positioning and construction clear.