You've got two questions here, let me answer them one at a time...
First, on the issue of building a preban from a receiver, BATF has issued an advisory opinion that to be a grandfathered assault weapon (see 18 USC 922(v)(2)), the firearm in question had to be completed with the ability to accept a detachable mag and two or more defined assault features [i]prior to 9-13-94[/i]. Thus, if you bought a stripped receiver that was never assembled, but was manufactured prior to 9-13-94, you are out of luck. If it was completed for 5 minutes and then disassembled prior to the ban, you are good to go.
Now, as for the issue of whether a letter from your friendly local gun store owner would 'count', it would probably suffice if it can be shown he did in fact own the gun prior to the ban. The problem is that there's no particular form of proof that is deemed acceptable or not acceptable in the law. Basically what you need is a form of proof that will make a prosecutor or a jury believe that the receiver was actually built into a complete firearm with all the required features prior to the ban. A letter from the manufacturer is pretty strong proof of that. A letter from a dealer is pretty good too, but probably not quite as convincing as a letter from the manufacturer.