Rather than a manufacturer model, it represents a single specific tuned car, kind of like the Deberti and Forsberg cars. It was built out of a 300TD diesel by AMG North America, who were the licensed dealer for AMG in that region, out of genuine AMG parts including a 6.0 4-valve crate engine. AMG themselves never put the 4-valve in a wagon, so this dealer-built car is the only one. This is also probably why it's named 'AMG Hammer' in the game, the nickname coined by a magazine review, when AMG would have just labelled it a 300TE 6.0 AMG.
The same owner who commissioned the 4-valve wagon also had a coupe and saloon built with the same engine. The wagon and coupe were sold as a pair to another owner (who loaned them to PG for the scan), while the saloon went to someone else, which might suggest why it hasn't been featured in Forza yet.
That black car is the other wagon built by AMG North America, with a 6.0 2-valve in it. Not an identical spec, but many similar parts, and probably circa 310bhp as even when AMG-modified the standard Mercedes heads don't flow brilliantly well compared to the bespoke AMG 4-valve heads.