Here are some pictures sent by a reader who identified himself as racer x. The first is an image taken from the ground level from the back of 231 Duffield Street looking down- click the image to enlarge. The wall between 231 and 233 Duffield is on the left:
Here are the pipes in the foreground of the photo above. Notice how they go behind the stone wall towards a brick wall (actual outer wall of property):
The photo below shows the view from above the false wall towards the property line. It shows a gap of about five feet:
Several advocates believe that slaves hid in this crawl space. I have spoken to archeologists familiar with buildings of the mid-19th century, and they had never seen any sort of construction like this.