Yep. My bet is that it's reported stolen. Maybe from some bored young punk writing serial numbers down from pictures he sees on the internet. Maybe as part of an insurance fraud scam too.
If it is reported stolen, you'll lose the gun and the money you invested in it. And get a refund for the tax and have to start over again from scratch. Hope you didn't pay much for it.
Was it used? New would be a lot easier to show that it's not stolen, but you never know.
I have been wrong before, and I hope I am this time too.