There is what appears to be a nice Italian bike with a Paris-Roubaix derailleur for sale now on ebay: http://ebay.com/
I would guess that the frame has been repainted, when the newer headset was added. The chainring is also a later version. If the cranks are indeed Duprat (the photos are not sufficiently clear for me to say anything about them), they too have been switched. The seller estimates that the bike is from 1950, but this is not the case as the writing appearing on the derailleur chain guide dates from no earlier than 1951.
Nonetheless a nice bike overall.
--
Steven Maasland
Moorestown, NJ