I hate to make accusations when I don't know all of the facts but the seller is advertising a factory XM177E2 which this is not. It might have all factory Colt parts, but as already pointed out, it has some serious issues with the date of manufacture on the receiver and the moderator. There is nothing rare about this gun except for the lore the seller has implied. It is a parts gun.
There are a lot of transferable M16's in the 5 million serial number range but most of them were built as 613's and more commonly as 614's which would have been slickside Air Force type guns. I've seen plenty of them at Class 3 dealers and at Knob Creek over the years.
Most of the transferable XM177E2's on the market are commercial model 639's and are so marked. Many had the moderator removed and discarded. I've seen them with and without. A close friend bought one years ago and it had a birdcage with the grenade ring but no moderator. Still, it is marked Model 639 and is the real deal.
I would just hate to see someone pay a premium for this particular gun knowing that it has been assembled by someone other than a Colt factory employee that would be retired today.