Here is the overall view of the valve access box: The valve is turned off because the pipe directly after the valve has come loose from the valve. I'd like to repair this, but I'm not sure what the grey/brown piece just below the valve is: Here are a few more shots of the side and the cap on the part that sticks down:
It looks like a backflow preventer. Its purpose is to prevent the water in the irrigation system from being drawn into and contaminating your house hold water supply. If the water pressure to your home were to drop the pressurized water in the pipe could flow into your home.
It's an old Zurn Wilkins backflow preventer. They're still in business.