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Are explosive grandmothers a common thing among them? In the society in general? Or was it just this one group that had one?
Explosives mounted on or inside a person's body are not a particularly new invention. It's possible that storing a bomb inside a person for use in desperate circumstances was, if not common, at least not unheard-of. Given the Boer's agrarian social structure, a old woman (well past childbearing years and probably physically frail) would be the logical choice of carrier.