This French rollergun was featured as part of an article on imported spearguns way back in March 1951. The gun was referred to as the "Supergun", but I don't know if that was its actual name or one conjured up by a local distributor of dive equipment. The operating principle is shown in a schematic which is taken from my series of "Basic Rollergun Diagrams". Note that the rubber bands retract back into the metal barrel tube when the gun is discharged and the adjustable position band anchor makes use of a slot in the bottom of the barrel tube to lock it off at various notches or "doglegs" located along the length of the slot.
I have never seen one of these "Supergun" spearguns, but I expect that the gun was a "sinker" as the barrel tube must have flooded as the bands have to be able to move freely in and out of the barrel tube and off course there is that slot running for some length along the bottom of the barrel tube. Stylistically the grip handle appears to be from a manufacturer that may have also been responsible for the "Doublette", a speargun that was both a spring gun and a band gun in the same underwater weapon.
From the forward position of the band anchor slots I doubt that this rollergun used any preload, but the same idea is at work that would be taken to more extreme levels in later decades. Here the alternate anchor positions weaken the shot.