You do not have to put any special markings on your receiver. I checked with ATF before I built mine. I did not put any special marking on it and they approved it.
I can't bring myself to spend that much money on a Mossburg. I have a Remington 870 Witness Protection built by Automatic Weoponry(now Wilson / Scattergun Technology). Remington is now making thier own aow for commercial sale. check with MidWest Gun Exchange in Mishawauka, They can find you one...