Yeah, that's an oddball for sure. I know the license built Perkins 4.154, it was imported into Canada in the Mazda B trucks and the Ford Ranger in the mid 80's. It's pretty much the same engine but with Mazda specific externals. What heat exchanger does it use? Just for future reference, ALL the Perkins part number beginning with USN or USM, are invalid. Since Cat bought Perkins, all part numbers have numerical references.