Parts
Plumbing parts: a model number in, the part that fits it out, with the cross-fit alternatives and the spec sheet that says so.
Each parts page starts from a model number and ends at the component that fits it, with the cross-fit alternatives named and the manufacturer document that establishes the fit cited.
Water Heater Anode Rod by Model: Which Thread, Which Length and Which of the Three Rod Types Fits Your Tank
Residential anode rods are 3/4 inch NPT with a 1-1/16 inch hex head. Rheem publishes 42, 44, 54 and 58 inch rods; Bradford White cuts segmented rods to length.