When you say six, you mean the assassin's mark, which was used to get Artell's armor. You need to enter the graves of six assassins in different cities in the game to get them. After all the collection, return to the castle villa and enter the basement from the study to get it.

But' Artur's clothes' is one of Ubisoft's rewards for genuine users. You need to take U point and change it on Ubisoft's Uplay. If yours is not genuine, you can only find relevant cracked files.

There are three other rewards: extra flying knife, auditore family assassin's tomb and wallpaper.