Have you had a MRI yet? Sometimes they won't show much unless an obvious anomaly - but good to have to be sure no other causes from the c spine etc.
If a cervie rib it should show, but possibly they didn't look for it, or missed it?
Also could be large bones or ligaments, unusual anatomy (we all aren't exactly alike after all -some unusual vascular /muscle configurations have been mentioned here)

But often the more difficult diagnosis will take more time and must rule out many other dx's before a positive TOS dx is made.

Be sure to read through our sticky threads up near the top.

How is your overall posture?
hunching shoulders or letting them roll forward just closes down the area that TOS affects so while you are working on a dx you can always try to work on posture , to see it if helps at all.
Same with head jutting or being forward of the shoulders - ears should line up with shoulders.
The weight of the head forward constantly pulls on the very muscles that we are trying to relax. So it should be balanced over our body in line with the spine.