I just made a call to BATF about this "mystery" license. The answer I got for the discrepancy in the number was that it is part of their filing system, and would be different that an older license. He had no explanation for why I received the new license, only asking me if I renewed my license. I told him that it was over one year ago, and I have the original that was sent to me then. Still no explanation for the new document, but he assured me that it would be my current, valid license. Still got me confused. Especially since it seems that nobody else has gotten one "out of the blue".
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!