Raimey,
No,I never have seen such a spanner,but it makes more sense than any kind of pressure relief system that vents the gas toward the shooters face. One reason for a 4 pin spanner could be that 4 small pins would be stronger than 2 small pins, when there wasn't enough room for larger holes in the bush." When you hear hooves, think horses before zebras",as the famous author Mr.Anonymous once said.
Mike