Whoops !
My first answer is incorrect!!
I was thinking about the cotton reel technique when I spoke about brachioradialis, so please accept my apology.
In the cobra technique the little skinny bit of muscle just above the biceps brachii tendon is the supinator and everything superficial to the supinator is the anconeus muscle.
If you scan slightly proximally from this location you will see the supinator disappear and then you can see the elbow joint capsule (annular leg and LUCL) coming around, over the top of the hyaline cartilage of the radial head and attaching to the supinator crest of the ulna. Just superficial to the joint capsule you see a tiny artery which is the recurrent interosseous artery and the muscle superficial to the artery is again anconeus.
Sorry about the confusion with my first answer!!
Steve.