I don't know and I don't care either way.
As for it being a signature... Well, I wouldn't say that. It was somewhat but the middle finger has always been more-so.
Sounds silly but he's almost turned sticking his middle finger up into an art in itself. Not that he was the first to do it lmao, but it's just become a part of his image. When other people do it, it feels childish or pointless but he like... I dunno, every look / image back in his prime felt crafted. All a part of his mr. don't give a fuck swell of appeal.