TBH, I'm not sure I would have recognized that as a Nazi symbol prior to a group of people I knew arguing about Death In June. Some were arguing that the band was fine, some were arguing that the band was pro-fash. The second group was using the band's iconography--including a totenkopf--as part of their evidence. The first group was claiming that it was satirical. (FWIW, Death In June had a long history of collaboration with Boyd Rice, who is an out-and-proud fascist, which is pretty strong evidence all on its own.) That was around 2010 or so.

If I hadn't been part of that, I can't say when it would have broken my threshold of consciousness.