I'll have to check my Greeners specifically, but in the vast majority of pommie guns, the safety acts on the triggers, not the sears. After discharging one barrel, a dainty trigger-return spring brings the trigger back to the unfired position, at which point the safety can be re-applied.

If the safety acts on the sear, then no way could it be reapplied after discharging one barrel.

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Just checked a couple of Greener 'Empire' shotguns in my employ, all allow re-application of the side-safety after discharging one barrel.