This is from the 1932 G.W. Baist map. You can find a copy at the Washingtonia Collection at the MLK Library in Gallery Place. You can also find a digital copy at HistoricMapworks Dot Com, and maybe the Library of Congress' American Memory site (but I can't seem to find a stable link). Now I realized I cropped the names of the streets off, so Square 521 is between P and Q Streets and 3rd and 4th Streets.
So where where the Bates Market/ Commerical Space that needs leasing, there was a coal yard there. Considering the amount of space the market takes up, not a particularly big coal yard. The Mosque I'm guessing is sitting on top another coal yard.
The reddish/hot pink in the map denotes a brick building. Those clear buildings in the rear of some brick buildings.... I'm guessing poop shacks/ outhouses. But that's a guess, they could be tool sheds too.