That was actually composed of at least 4 trades of near equal size - I said "at least" because as you can see Quotetracker only captured real time raw data at 15 second intervals - and note the matching price/bid/asks, so these weren't end of day market sweeps, they were "real" trades:
Yes I saw it as well. But may I suggest the following for COMPLETE analysis of AH or at-the-bell postings.
The market has 20 minutes after the close to reconcile the day. That could be unposted trades, transaction errors, and other "mis-steps" (purposeful or not) that occured through the regular days transactions.
That sais, could it have been a 1.5M block? Yep. Could it have been something that happened during the day that got posted just at the close? Yep again.
My point. Please evaluate the AH and at-the-close (16:00) activity with a degree of cynicism and NOT for it's face value alone.