They used to be marked "M4 Carbine" but appears that they have switched to "Model DDM4". If your s/n is "DDM4XXXXXX" then your model is likely "DDM4".
Also, your first post says your Form 1 info was what was in question but your last post says it's a factory SBR which should transfer on a Form 4. Did you get it mixed up or was the wrong form submitted?
You are correct. I just pulled out my copy and it's a Form 4.