Just sent you plan's on the receiver for the hole placement.
As for the receiver, has the receiver been painted or refinished and the hole is drilled, but just covered. If not, them you should be able to send the receiver back to bushmaster to have them drill the hole.
As far as the letter that you receive from the dealer, I would want a little more than that if I went to court. What you need is a bill of sale for the completed firearm that shows that it was assembled before the ban or a letter from Bushmaster stating the the rifle was assembled as a complete rifle before the ban. Remember, even if the receiver was produced before the ban, It had to be a completed rifle too or your rifle is a post-ban. The only thing you have now is a stripped receiver that was sold after the ban.
What I find strange is the fact that pin was never drilled. I don't believe that bushmaster would have sent a completed rifle out without the bolt catch. And, had the rifle been assembled before now, surely someone would have sent it back to be repaired.
Your best bet is to contact bushmaster and find out the info on the receiver.
Here is a list of their receivers in regards to pre/post ban.
Bushmaster 800-998-SWAT
Below L051000 - Assembled Pre-Ban Rifles and some pre-94 lowers.**
L051001-L063000 - Mixed Pre-Ban Assembled Rifles and Pre-Sept 94 Lowers (Call to verify)
L063001 and Up - Post-Ban
P.s. If it does turn out that the receiver is a post ban, then you can get your money back. With the letter that he signed, you have him by the short hairs.