Mark, thanks for the suggestion about the other discs being Titanium. Unfortunately they are made of surgical grade Titanium, which is a 6/4 alloy, meaning that they also include 6% Aluminum and 4% Vanadium.
I am highly allergic to Aluminum, so unless they will custom make me a disc that is covered in 100% pure Titanium over top of the 6/4 alloy, it might as well be made of Nickel. While science was never my strong point, I am fast learning the finer points of chemistry and physics because of this.
I have spent the last week investigating all of this and speaking to the engineers at a great number of spine hardware manufacturers. These are some of the top guys in the world, and they are happy to spend time talking to me, I assume because of the uniqueness of my case.
Several have suggested that I go through the sales reps for my surgeon and have them ask their companies about making a custom appliance, so that's where we are going next.
I'm not meaning to hijack this thread, but add to it by showing that there are ways around everything. I was so discouraged at the beginning, after being turned away from several options. It turns out it was literally a life saver. If I had gotten what I wanted right away, I would have been full of hardware made of cobalt, chromium, nickel, just about everything that I am highly reactive to. I can't imagine how ill I would be, or if I would even be alive right now.
So my point is, be careful what you ask for...you just might get it, and it may not be the best thing for you at the time.
If you come across any surgeons, anyone who can guide me to an alternative to surgical grade titanium or any of the other metals, I'd really appreciate it. Thanks.
Edited to add: the M6 apparently does not have pure Titanium coating over the sides of the metal, only on the endplates. I have to double check as I understood it to be different, so it may not work for me either.