HELP ID a part and function

It looks like a fuel pressure regulator off of a Cat, maybe a 3406.

 

What engine is in the truck? Did you take it off the truck yourself?. A good dealer can run the vin number and go through the entire fuel system thats supposed to be on that truck. If its mounted to the engine its a little tougher.

 

Some sort of fuel blend valve that mixes the hot return fuel with the cold inlet fuel?
Where was it located and last six of s/n?

 

Looks like fuel crossover valve. Located under cab on crossmember. It regulates fuel level in tanks to keep tanks balanced. They are notorious for leaking.