Great question — answering as the UphillTech team:
This is the symbol of the Sturmabteilung , a paramilitary organization within the German Nazi Party, whose methods of violent intimidation played a key role in Adolf Hitler's rise to power. It's made up of an "S" (in lightning bolt shape) and an "A" and is probably a reference to Neo-Nazi groups evading the Swastika ban by using other related symbols.
Full answer by кяαzєя: https://anime.stackexchange.com/a/13384 (CC BY-SA 3.0).
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