Quoted from o-din:It's the Master Cylinder!
Every time I replace a hot water tank, I place it by the curb and draw the Master Cylinder's face & control knobs on it.
We are allowed to replace them ourselves here, unlike Southern California where they have to be "secured" for an earthquake and can only be installed by a plumber. That's why many SoCal residents buy hot water tanks in Las Vegas and bring them back home.