I’d guess a dpf/catalyst off of a diesel pickup. Doesn’t look to be a Ram one though….
I think that is what it is, and from the 2 I have seen, off a newish Ferd diesel, the delete boys are at work in my area, I have only seen them off the trucks, but they DID have a bolt together flange ... and I am told they bring BIG bucks from the guys who got caught without them.
Good to see you got your truck back, what kind of condition is it in?
If the truck is unhurt, the thieves might have left you some payment.