Exciting topic Spun Ducky
My guess is that it's none existing on lower reqs and the cap was q7/16 like q0/9Armor also should not exist.
So the basic rule " req X+ 8= Max armor (for the req) "should be only broken by the q7/16 version.
But GW is full of surprising, quirky and insane rare Items so let our mind be blown and the end maybe will read here with excitement what the prenerf-pros think/say
There are r7/16 armor shields. I just saw two of them. One was a dual defense and one was -2/+30 or something.