External differences between cat 6nz and mbn engine?

Trying to find out what engine I have without paying someone to read the ecm. I was told by the person(dealer) I bought the truck from it was a 6nz. Talked to previous owner today and he said when he bought the truck the engine had either had an out of frame or engine replacement. He said this because the engine had been painted out of the truck, defining line between engine and transmission. Anyway that has got me to.thinking, what if they replaced the original 6nz with a mbn? How could you tell externally? And there are no tags or plates anywhere to tell me what it is? Also got me concerned because of the fuel mileage and it does not pull like I think it should.

 

Well I have looked and looked. Only thing I can find is the block casting. 125-8850. Which doing a little research, says it is a c15,.6nz 3406e block. Can anybody dig up anything else? Also noticed on the ecm there is a decal that says this engine is epa/carb certified tampering with fuel settings will result in fine. Why would it be carb certified if it is a 6nz?

 

here is a picture of that decal I was referring to.

 

has aftermarket turbo on it....borg warner 177148