If you look the door tag, you should see two things
The first thing is
3 is the first digit of the VIN, 3 means built in Mexico
the 2nd thing,
at the end of the Production year, the Saltillo Plant usually finishes building all the Heavy Duty Rams by the month of June.
So all 1500 Crew Cabs built in Saltillo will have a production date of late May, June, July, August
The Assembly Plant in Warren Michigan usually have excessive amount of Build Orders left to build.
At Ram Headquarters, they have made the decision in the past to divide up the number of orders, between Saltillo & Warren Plants.
The Saltillo Plant does not have the Metal Stamps to make anything but Crew Cabs & of course Standard Cabs.
They can not build Quad Cabs down there.
Some of the Crew Cab 1500s that are built in Saltillo Mexico, you may notice that the front end suspension & brakes may come with Heavy Duty parts
I have not heard of any 5th Gen Rams being built in Saltillo