I know punk!sirius is a thing most people are really into. But the clothes he's wearing in Order of the Phoenix are likely the clothes he owned before being sent to Azkaban. There's no way he'd wear his father or Regulus' clothes. No chance to go shopping. He had to wear his old clothes, probably wanted to wear his old clothes as well. Sirius Black in his velvet suit doesn't look very punk rock to me, especially not 70s British punk rock. But during the late 70s and the early 80s you've also got disco. Saturday Night Fever came out in 1977 so it's a toss up on if he saw that film or not. But seeing the film is not a requirement to be into the disco scene. Seriously let yourself think about disco!sirius for a moment. Him listening to the Bee Gees, singing Stayin' Alive at the top of his lungs in the shower, going out to muggle night clubs and tearing up the dance floor. Sirius Black may have had a punk attitude but I think he had a disco soul.
