Fred,
I don't know if you marked those two cases or not but have you checked them to see if by some odd chance they could be sized differently? Maybe, with a capitol M, the shoulder got pushed back a few thou? Just another thought.
What about primer pocket depth on those cases, since they were the only ones to missfire. Or, from what I've read, a possibility the primers were NOT seated completely. This, I'm sure you know, causes the firing pin to fully seat the primer on first strike and make it fire second time around.