Rather late on this thread, but since I have recently acquired a ca.1960 Helyett Speciale frame (currently being stove enamelled), and having researched it recently I thought I may add the following :
The Helyett team were incredibly successful during the period from the late 50's to early 60's winning many major events including TDF. Anquetil being their star rider.
The 'Speciale' was the team machine and the livery was a dark green metallic frame with the Alloy Helyett head badge, depicting Miss Helyett - the opera singer. The frame had Nervex lugs that were specifically made for them and were not used (to the best of my knowledge) by anyone else. My frame was a puzzle when I obtained it. The bottom bracket was English thread and had a TDC fitting with Williams B100 5 bolt cranks fitted with TA adapter and chain rings. Odd for a French made frame ? Further digging revealed that Helyett (Evian involved here also ?) had a sales operation in N London offering these machines. I suspect that they imported the frames and built up the machines in London. They may have insisted on having BB threads to suit the UK market ? Best guess only.
Plan is to build the frame up once restored with period correct components. Acquired so far : Stronglight 49D crankset, Normandy Competition HF hubs / Atom skewers, PIVO stem, Ideale 88 saddle. To find : period correct rims, correct Simplex gears, Pivo (probably) bars. Should be ready to ride in a year from now with luck !