Soap my arse!
I'd rather not if it's all the same to you!

To quote from my source (The ACO 1923-1982 book):
".. et Serraud-Giraud Cabantous, leurs dauphins, prient le ciel pour que la soudure au savon de leur réservoir tienne jusqu'au poteau"
which basically translates as something like they were praying to the sky that their "solder" made of soap would hold watertight.
And it obviously did because they finished 2nd overall!!!
Presumably it was used more as a moulded bung rather than physically soldered!