"Originally, Slave I was a police vehicle from the asteroid prison Oovo IV. There were only six of the model in the entire Galaxy, all on Oovo IV.
While on the trail for the Dark Jedi and Bando Gora leader, Komari Vosa, Jango Fett's trail led him to capture Bendix Fust for the crime lord Sebolto. After tracking Bendix Fust to Oovo IV, and as he was about to capture him, another bounty hunter had beat him to Fust and began a prison riot. After fighting his way through armed guards and prisoners, Jango had made it back to his ship, Jaster's Legacy. Unfortunately, the rival bounty hunter had attempted to slice into the old ship resulting in its malfunction and permanent shutdown.
Acting on a tip from a prisoner, Jango made his way into the asteroid prison hangar bays and stole a Firespray-class vessel along with the rival bounty hunter, who was known as Zam Wesell. During their escape, Jango launched several missiles into the hangar bay and destroyed the other vessels, making his the one and only. Sometime later, Jango would name his new ship, Slave I. It wasn't until Kuat Systems Engineering reintroduced the line several decades later that other ships of the kind would be seen. "
and later Boba would get upgrades until the point where i believe he says only the seat is original in the Slave 1. also it says in its description MandalMotors did some modifications on it to and they were Mando techs on Mandalore and of course SW databank doesnt hve a single thing on this matter so take Wookies word for it.
"I will bet all my Star Wars guys. Whoa, whoa, whoa, wait! Except Boba Fett. No matter how sure I am, I never risk the Fett man."