What's missing?

I have been playing with this new NYC.gov online mapping system which allows you to see Brooklyn back in the day... 1924 to be exact. So what important landmark is missing from this photo of our beloved South Park Slope/Windsor Terrace? On the right is Prospect Park, towards the bottom left is Green Wood Cemetery. A hint: Construction started almost three decades after these aerial photographs were taken. OK here's the answer.

I thought I would zoom in over IMBY Central Headquarters... That large black rooftop is the Park Slope Armory .