Thanks for the reply, Feel free to check the spec to see if I am reading/interpreting it wrong. My understanding though is as stated in my opening post that this zone has been identified and allowed since the 1991 version of the red book spec.
Your concerns about a 'working' stack ring I do believe are addressed in the red book spec as well as in the information provided in the opening post. The read out side stack ring zone is from 33 to 39.5 mm in diameter, while the label side stack ring zone is from 33mm to 44mm in diameter. With a normal stack ring being only 1-1.5 mm in width, there are multiple combinations of raised rings on both sides that would all nesting and stacking.
Regardless, amongst the few people who are reading this thread, none of us have seen one. The only fringe case of existence of a label side stack ring on a "CD" that we have come across to date is on a Dual Disk (yes I am aware that these do not conform to red book in the first place). The DVD media on most Dual Disks has a read out side stack ring and a label side "groove." When played as a CD it has a label side stack ring, and a read out side groove.
If anyone else finds one, please let me know.