Your water heater has two 4500 watt elements so water heater is 240Volts, and water heater is rated 4500 watts. Both elements are not ON at same time, and this is called non-simultaneous operation.
The element might have been smaller in length, but it will work fine.
Why is it smaller. I don't know. Elements are different from manufacturer to manufacturer.
The 501 does not use a special element.
Local box stores and hardware stores sell elements in water heater department.
The wattage of element is printed on end of each element.
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