The question has arisen because the SSR says "Dummy stack", not "Dummy counter", and Will's original question was asking: what exactly is a Dummy stack?
As he said, it's not really defined anywhere. The best I could find was A14.23 (talking about a concealed stack as a Sniper target):
If it contains [no eligible targets] (i.e., is a Dummy stack)
Also A12.11 implies that a Dummy stack is one that doesn't contain a real unit:
the DEFENDER may force the ATTACKER to momentarily reveal a non- Dummy unit in that stack to show that an actual force exists there. If he cannot ... the Dummy stack is removed.
My understanding was that a Dummy Stack is a stack that consists of only "?", the example I posted has a real counter in there, therefore it's not a Dummy stack, therefore the 5/8" "?" doesn't have to take an NTC.
OTOH, I take his point (in his example C) that you can have concealed and unconcealed units in the same hex, and they would probably be considered as 2 stacks. But in the screenshot I posted, everything is concealed, so is it considered 1 stack or 2?
If it's 1 stack, then the 5/8" ? doesn't take a NTC because it's not a Dummy stack.
If it's 2 stacks, then it does, because one stack is the concealed leader (which has to take an NTC because it's Infantry) and one stack is the 2 5/8" ?'s (which makes it a Dummy stack).