1" screw in elements dont exist. All screw in type water heater elements are 1.375" diameter. They all are. You can use a 1" straight thread nut from Bargainfittings.com and the supplied seal to install the element in a kettle. But all elements are the same size, 1.375"
1" couplings fit, it is all in how they are measured. Remember as Bakins said, they are STRAIGHT thread, not TAPER like most pipe threads.