Just chanced on this fibreglass pole spear from Taiwan made under the "A-Jiou" brand name. The trigger operates by hooking over a small washer which means the draw is fixed, however that is not really a problem as you usually cock these things to about half-way distance on the pole. Can be used with more than one band.
https://www.facebook.com/A-Jiou-Pole...1908624669060/