That hunk of beef would be Beef Chuck Underblade Center Cut Boneless. It is cut from the serratus ventralis which is a very highly marbled muscle, and is of adequate tenderness. If that muscle were to be cut into separate steaks, they would be called "Denver Steak". See the attached link....
this is my first time cooking flat iron steak i bought two lbs and coated with Montreal steak seasoning and placed it on the rib-o-lator until internal temp was about 125-130 and then placed on a hot grill to reverse sear. this is a great steak, it was tender and very juicy, thanks to the rol of course. the small one is my wife's.
sorry for the blurry pics
i cooked them for about 30 min at 300-325 and they tasted great.