The Tour of Flanders is littered with stretches of cobbles but their somewhat milder surfaces(?!) apparently didn't warrant the use of one-off machines that we usually see at Paris-Roubaix.
In fact, 2004 Paris-Roubaix winner Magnus Bäckstedt (Slipstream-Chipotle) left the start in Brugge with his usual Felt F1 Sprint, complete with a Zipp 303 carbon tubular up front and a slightly deeper Zipp 404 in back. Save for the custom paint to commemorate his Swedish national champion status, there was little to distinguish his bike from those of the rest of the squad.
Slipstream does have Paris-Roubaix specials, though, and those bikes did still make it out on course. But they were mounted atop the team cars as spares for the day. The current Belgian weather promises to make this year's 'hell of the north' a decidedly messier affair than last year's uncommonly hot and dry running, and from the looks of those 'spares', Slipstream riders should be ready to go.
The Paris-Roubaix Felts utilize a mix of F1 SL and F1 Sprint materials for extra strength and stiffness without too much added weight. Kevlar patches are also applied to the top tube for additional impact resistance from errantly spinning handlebars.
As is typical for the genre, there is also more tyre clearance all around courtesy of longer fork blades, rangier seat stays and chain stays that are slightly indented right where the tyre passes through. Unique rear dropouts decrease the bottom bracket height a bit for more stability. A 50mm-rake carbon fork offsets the longer rear end to maintain overall weight balance and also yields a longer and more stable wheelbase.
Longer-reach Shimano R600 brake callipers are used to accommodate the increased clearances but it looks like the rest of the componentry (save for the wheels, of course) will be standard team issue come this weekend.