No offense to anyone that posts this as fact but…
This isn't in any way confirmed. Leakers haven't found anything in the game files relating to the legacy weapon. With both of the past ranked reward crates the gun was found in the files and that was how we first got news of it. The By15 speculation is based on a random LATAM social media post that claimed to had been given the gun already. If the gun isn't in the files at all how can someone given it? The post has thoroughly been debunked.
Until someone like leakers on Duty comes out with the news please don't believe it. Even if they come out with it don't take it as a complete fact.
Please don't get too excited for it to be a By15 because I don't want anyone to be disappointed when it ends up being a fuckin dingo or something. Well, I mean you can be disappointed in getting a dingo but that's a different conversation.