My FEL has a serial number just like the tractor. In fact most of the implements for my tractors have serial numbers. My current BX 2660 was "broken down" and insured as the tractor and mower with values for each. Whether Kubota sells the unit as a package is not relevant. They sell the entire tractor part by part. If the tractor were to "roll" into a pond and not be serviceable the insurance company would most likely "total" the tractor and sell the "salvage" Insurance is regulated state by state. A policy written in Texas is not necessarily the same as one written in New York.
I have had a tractor stolen that my agent did not add to my policy. Fortunately the loader was on the policy and I had added insurance at the dealer level for the tractor and loader. Now the fight was on to see which company would not pay for the stolen loader. And that is another story.