If Diversified Machine shipped him something that the ATF already considered a suppressor, it does not matter if he had his stamp. You cannot use a form 1 to order in a complete SBR, for example; it is permission to MANUFACTURE, not to transfer. If I ship you a Colt 6933 with no form 4, the ATF is going to be busting you and me for it even if you had a form 1 to build an SBR.
The "lol **** the ATF crowd" really does not seem to understand the nuances of the law. I'd strongly advise consulting a lawyer before blowing this off.
OP did not buy a complete suppressor. Machine work by the OP was required in order to arrive at a complete suppressor. The OP purchased said parts in good faith from a reputable(arguably) company.
A Colt 6933 without a stock attached is nothing more than a pistol, is it not?