Quoted:
The flash hider is timed with a peel washer rather than a crush washer, so it's a relatively old barrel- without checking, I would say that the crush washer pre-dates *any* kind of extended feedramps, and there's no way that that's a factory configuration, as others have said.
~Augee
All the 80s and early 90s era pre-M4 700 series carbines and early M4s used peel washers and they had the extended feedramps and M4 feedramps. According to the USGI Colt Carbine Parts Variation Guide tacked in the Retro Forum, the crush washer was introduced circa 2000. For instance, I've got a late 90s/early 2000s Colt 733 upper with A1 sights and case deflector, marked M4, with M4 feedamps, and it has a peel washer.
http://www.ar15.com/forums/t_3_123/296919_USGI_Colt_Carbine_parts_variation_guide_Edition_IV.html
Just by eyeballing the pics, the extension on that barrel appears to be the slightly shallower, pre-M4 extended carbine ramps. My guess would be someone wanted M4 feedramps on their rifle really bad so they had that extension installed on that barrel to mate it up with that receiver.
OP, any good pics of the index pin and where the extension mates to the actual barrel?