AFAICR Trinitron models have separate vertical and horizontal focus controls. Follow the single highly insulated lead back from the flared part of the tube to where it originates - that should lead you to the focus pack. I'm a bit out of date but I don't think this component is necessarily on the main circuit board, instead it's usually mounted on the chassis metalwork. If one of the controls is marked "screen" then you probably just have one focus control but I think that's unlikely.

You will find that both controls affect one another so my advice would be to make small incremental adjustments. This is best done using a crosshatch display using white lines on a black background.