Pete, I think that the slider have the main centering role in Taimen spear gun. O-ring that holds the slider in place all the time until the shaft tail leave the muzzle helps that function of the slider. Without that O-ring slider might left the gun too early and there would be no centering in the most critical time.
In Tomba vacuum kit the only one O-ring on the shaft serves for maintaining vacuum and for holding in place the cone ring for centering the shaft. The shaft is in center with tolerances +-0.05 mm, all the time during shooting. I suppose similar tolerances are in case of Taimen too. With salvimar kit that tolerances might be +- 1 mm (or maybe +- 0.5 mm in best case)?
In Tomba vacuum kit the only one O-ring on the shaft serves for maintaining vacuum and for holding in place the cone ring for centering the shaft. The shaft is in center with tolerances +-0.05 mm, all the time during shooting. I suppose similar tolerances are in case of Taimen too. With salvimar kit that tolerances might be +- 1 mm (or maybe +- 0.5 mm in best case)?