That's a lovely frame you have bought, and I have no doubt that it came from a respectable builder. However I doubt that you will ever find out who built it as it doesn't have enough 'different' distinguishing features, although the short top-eyes are unusual for a 50s French road frame.
The Nervex-type lugs are in fact Oscar Egg Super Champion Model SC3
ones...the most popular of that brand's model range...and they tended to
predate the well-known Nervex Pro Mkll lugsets that we all know and either
love or loathe. I reckon, but have no way of proving my thesis, that Nervex
took their inspiration for the Mkll headlug from the Super Champion one.
>From what I have learned from decades of collecting, no large scale
manufacturer ever used the Oscar Egg lug sets..just the small artisan firms.
>From memory I recall that the Oscar Egg lugs were among the first high
quality pressed steel lugs to come on the market...whether the Nervex Serie
Leger predated them a little or arrived alter I'm not too sure. However more
Serie Legere turn up on the frames of large manufacturers than do Oscar Egg
SC ones. One explanation for this preference by the larger manufacturers
could be that Nervex ( or Ste Franco-Suisse as it was called at that time)
used to produce the full kit. ie not only did they make lugs, but also
bottom brackets (which would be matched to the angles of the frame) but also
drop-outs, albeit pressed steel ones..and braze-on parts. On the other hand
Oscar Egg just did sets of three lugs...although there was a range of about
five or six very attractive models Urago of Nice was one of the medium-sized
producers of high quality frames who used Oscar Egg lugs...as well as Nervex
lugs. The superb road and track frames produced by Special CNC, in Paris, in
the early 50s more often than not used Oscar Egg Super Champion lugs.
The builder of this particular frame has selected to match a Nervex bracket to the Oscar Egg lugs instead of a Gargatte, Dardenne or RGF, and has chosen a fork crown that looks like an earlier Wagner one. Urago also used this type of crown, as did many other artisan builders.
It seems to me from trying to magnify the photo of the gear-side rear drop-out, that there was originally a gear hanger attached. and it has been cut off and filed smooth.
As for tubing...well it is likely to be Reynolds..very popular with French builders, but might also have been VITUS. You might just find the brand stamped into the steel some place - fork blades, steering column. The frame is neither Special CNC, nor Urago...but is appears to be quite a pleasant looking and well constructed affair. I bet it rides a treat.
Norris Lockley
Settle UK