06', just a muffler. Looking at the inlet piping, frame and fasteners, it MAY be original, but your truck would have to be very low miles. MXS is a keeper. Don't let that engine go. It's worth 5 times what the rest of the truck is worth.
I agree it looks like a muffler given the Donaldson logo. Call KW for a price on the replacement, if you pass out from sticker shock you know it’s a Converter. You are in the year range where it could have one, or had one……
On a Mxs if that is the original muffler/catylist would be a catylist on it, they were made by Donaldson. I've never known anyone to put one back on one when they need replaced or get plugged up, new catylist is $4200+ or you can get a regular muffler for a couple hundred.
Only way to know for sure what it is is to run the part number on it or look inside of it.