Trigger or button to fire guns.
Older aircraft generally had higher stick forces, thus to fire the guns with a trigger it would be necessary to remove a finger from a stick. Three out of four fingers remaining could increase the relative control force, and decrease the accuracy of gunsight tracking. No electric trim.
Wheel hand-grip and gun firing button maximised available control force, two hands, fidelity / harmonisation of pitch / roll control.
Later aircraft had lower stick forces, servo/power controls, and elect trim; these aircraft favour a trigger on a hand grip, less control force required, and also enables additional switch functions - thumb trim switch, simultaneous camera finger button.