Off base is not the right word. It’s just that anode thread sizes are standard throughout the industry, so a Smith anode will fit a Rheem heater and vice versa.
They are all 3/4-inch port, and that is 3/4-inch inside diameter. The OUTSIDE diameter can vary quite a bit. The hex nut will have the same diameter on all, but the anode itself might be as skinny as 5/8-inch to as thick as .90-inch.
Thus, there will never be a form such as you describe.