A French patent by Philippe Tailliez shows a design that he applied for in December 1938, but it was not published until 1940, which of course would have been in wartime France. It is one of the earliest that I have found for rubber powered guns and is contemporaneous with the Le Prieur band powered speargun of the same year. Tailliez was a member of the famous "Bandol Trio" with Jacques Cousteau and Frederic Dumas. Before they got into developing and testing their Aqualung demand regulator they were all keen spearfishermen. The patent is in French, but here is the gist of it.
The hook 29, axle 30 and trigger bar 28 all form one piece which can be lifted out of the metal supports 4 (two parallel plates) through the opening shown to the upper rear. The gun uses "motor elastics" for rubbers 6 which appear to be of square section. They are trapped at the front by a muzzle plate 7 secured by the wing nut 19 which screws onto an embedded thread piece 17. The front of the barrel is slightly curved where the rubbers run to allow them to wrap around without being cut. The "wishbone" is a leather part 23 riveted around the two rubber bands by formed loops 24. The leather band in turn passes through a rectangular hole 26 in metal plate (figure eight shaped) piece 25 with a round hole for the hook at 27. The gun is loaded by detaching the trigger bar and hook assembly and engaging it in the "figure eight" plate while the other hand holds the gun at rear handle 2. When pulling the rubber bands up a foot is placed behind the front handle 3 to brace the gun. The rubber are drawn up so that plate 25 passes below the right angled tips of metal fingers 13 located on either side of the barrel, and then the trigger hook assembly is reseated in its rear mounted supports. The spear is loaded with the spear tail being trapped in the fold of the leather band due to the tension provided by the two rubber bands, in fact that is why he uses two bands. The spear, a plain metal shaft with a simple point, is held at the front by the lyre shaped clip 22 to prevent it dropping out of the open muzzle slot 21. Squeezing trigger bar 28 down rotates hook 29 up until the fingers 13 catch the metal plate 25 and drag it off the hook, which then allows the rubber bands to pull both it and the spear rapidly forwards. The gun is basically similar to a slingshot in its operation. Tailliez says a spearline can be attached, but does not show one in his drawings. Similarly the text mentions another lyre shaped clip to retain trigger bar 28 to handle 2 after firing, otherwise the assembly could fall out, but it is not shown. An interesting early design and probably actually used by Tailliez, unlike some other early patents which look to be designed by land lubbers as they often would not work and would be impractical to manufacture anyway.
I imagine that there are one or two of Tailliez's guns in a museum somewhere. Modern readers need to remember that spearguns for underwater fishing had only existed since 1937 and back in those days spearguns were an exotic concept given that divers hunting underwater with mechanical weapons had previously been confined to the pages of science fiction, namely the gun toting submariners of the 'Nautilus" and the 1933 "Nautilus" gun of Commander Le Prieur, which being somewhat too powerful, blew most fish apart!