In the Underground Harbor, Elliott, Will, and Drake found the model soldier in a room. Drake first thought it was rigged to something, as it was the only thing on the table. Later on Elliott noticed it was now painted to look like a Limiter soldier.
The model was an accurate depiction of the Wellington, the Iron Duke. It was made by a person with extreme skill and experience.
It is unknown if Wellington was really a Styx, or Eddie just thought he was clever enough to be one.