I'm pretty sure this was a feature common to only the Eastern 757s, as I've never seen it anywhere else.
As mentioned already, the older US 757s hall have it, N600AU through N609AU I beleive. Those are the Ex-EA birds, the rest are US new builds from later. It just also so happens that all HP
757s are Ex-EA machines as well. I read about that fairing several years ago and its come in handy spotting 757s at PHL
when you can't see the reg.
Actually re-reading some posts seem to make it clear it wasn't there when newly constructed. In any event, whatever that "thing" is, it appears only on Ex-Eastern 757s. I assume since both US and HP birds have it (and did long before the merger) it was installed late in their EA career.
Actually, its sort of ironic thinking about the history of those birds. After EA
closed up shop, these aircraft were split up, and now will be flying together again under the new US.
[Edited 2006-01-06 02:59:16]
I think Big Foot is blurry... It's not the photographers fault. There's a large out of focus monster roaming the countryside.