CJR - I have seen this a lot on Amex, though mostly with KRY. The only thing I can tell you is that several times I placed a "Market on Close" sell order when I was trading KRY and didn't have time to clear my positions. (This was also when the close was typically a few pennies above what you could get manually with last minute jumps)

Anyway, in each case those shares posted to my account anywhere from a few minutes after close to as much as 45 minutes later and never showed as individual trades but were at the price of a group that posted at the same time in AH. My broker explained that these types of transactions often settle as groups usually within the first hour of AH.

Also, ECN books sometimes don't settle right away, which is why during the day you sometimes see odd trades of decent volume but at a price outside the bid-ask. This also happens in AH but usually pretty close to the close.

Bottom line is I would say 99.99% that this is one of the above scenarios and not an actual trade AH. I would find it hard to believe there could be a matched buyer-seller with over a half million shares to trade AH.

It could also be a cross tade but I am not familiar with how those work.