The 50's & 60's Mavic rims I have seen have eyelets a a red diamond logo and don't appear to have a serated braking surface.
I have found an image of a blue Mavic logo on a 650b rim but it still looks different to my fragment.
Did look at Nisi but they have a winged logo a different braking surface.. also they are manufactured in Italy.
I feel the rims are original to the bike along with the other components all of French origin..ah with the exception of the rear Campag skewer...gotta get that sorted.
I've finally cracked the rim mystery just as the case was starting to get cold. Starting looking at other French manufacturers on Velobase to see if rims were listed and came close with AVA, but not a match. Then i had a hunch and remembered Pivo who make bars and stems, but no rims listed on Velobase. Next i just went with my gut instinct and did a crazy interweb search for Pivo Rims....boooom.... result.
Cheers Scooter, have a look at getting them on Velobase. Not got much more information on them though except width is 21mm, can't give wieght as I won't be de-lacing them. Other than that, all I know about Blue Ribbon is beer related....hmmm strange that.