If it is a corporate weapon, you should have a copy of the Form 4 and corporate letterhead identifying you as an authorized corporate representative to possess the weapon. It would be a good idea to have that weapon specifically identified by model and serial number on the letterhead just to cover your bases.
The likelyhood is that you will be better prepared and better informed than anyone who might ask about the legitimacy of the weapon. That includes most ATF agents who don't confront this shit every day. Speaking from experience, if you obviously know the regs better than the person questioning you, they are likely to move on.
Better safe than sorry.
Saleen