Hi Captain Haddock,
You hit the nail on the head with ARROW; as I'm sure you know the W is for Weight (and balance). If you haven't got the manufacturers W & B manual available, then the paperwork is not complete to fly the aircraft.
I know that the reality is that it's often been lost in an office somewhere as everyone uses the computer or simplified trim sheets, but's that's the legal answer. In practice, of course, it's particularly important to have the original for an acceptance flight as one of the things to check is that the computer programme/trim sheet has been made up correctly for this aircraft!
Either way, you need to create a fuss with the relevant people to get them to find it.
Have fun,
BJJ