Is part of this regulator missing for the sprinkling system?
Just everything that makes it a regulator.
mine has a large brass dome on the top of it …looks like it might be missing there
Looks more like a backflow device missing the normal cap. Any pictures from another angle? This looks similar. http://www.watertips.com/images/Watts800QT.jpg
I agree, it’s most likely a backflow preventer.
I amend my comment, it could be a low flow vacuum breaker. Made for use with lawn sprinklers. Stehpan and sparksnmore the picture isn’t very telling but I’m sure of one thing – the insides are missing.