I posted this in regards to a reply to Larry's first run in with the flat iron, I thought it might answer G Dechaine's inquiry regarding making your own FI:
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I found some "top blade steaks" at Publix yesterday that were less overpriced than the rest of the steaks, so I bought a couple packages - about a pound and a half total. I recalled, and confirmed from a google search, that top blade steak is sometimes used as another name for "flat iron" steaks. Come to think of it, they do look a little bit like the bottom of an iron.
the flat iron (infraspinatus) comes from the chuck top blade, but only makes up a small part of it. I talked to my butcher about it, thinking I could buy the subprimal and get a steak and a chuck roast. He talked me out of it, on the grounds that one large roast (that he ground) yielded a small percentage of steak, and that he had some flat irons in his case for 3.99/lb.
On chance alone (if the whole subprimal was cut into steaks), something labeled as "top blade steak" most likely would not be a flat iron. From the sounds of your steak, you did get a flat iron, so maybe they harvest them from the primal and grind the rest.
the infraspinatus has a sheet of connective tissue running through it. One of the possible explanations for its name comes from this connective which may have been thought of as "tough as iron." Due to this, the muscle has to be butterflied and the connective tissue removed. The steak ends up being fairly thin as a result. Other than the shape, looking for these clues will help you to identify it.
j biesinger
nickel city smokers
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this link shows the chuck top blade, where the infraspinatus can be found:
bovine myology
The only other pertinent info I have on harvesting the flat iron from the top blade comes from an out of print book by merle ellis. Its what started me on the whole flat iron business. His directions start with a small section of the top blade, so its kind of hard to make heads or tails of it, but I suppose if someone else wants to see it I could be talked into scanning the pages.