Hi Mike,
this certainly is an os acromiale.
The best way to look for this is to scan the AC joint in the standard way by straddling your transducer across the joint with one end on the acromion and the other on the clavicle.
Then turn your transducer 90 degrees and slide it laterally so you are on the acromion.
Then slide the transducer posterior and you should see a solid bone of the acromion. If you see another articulation in the acromion you have found one!
Steve