Looking through my Protools disks I have a version of 5.2.1 that came with my mbox, but I am not sure where the serial is or if you even need one with that disk, cause I never used it. Back then I was on windows but now i'm on mac. I do have all of manuals so maybe if u know a little more than me, I can maybe help you out with it. And just curious why would you need 5.1?