"If it was assigned a new number from the BATFE it would have a ATF prefix."
Not necessarily. I personally know that during the 1980s police departments were allowed to re-number, and sell, guns the SN had been removed from. I owned one. I removed the crudely stamped # (TC508) and re-engraved it in script. I refinished and nickel plated the gun and engraved the slide "Custom built by Bill DeShivs" I sold the gun to a world famous, huge, fat wrestler via an FFL. Two weeks later said huge, fat wrestler was arrested in Pascagoula, MS for gunrunning. ATF visited casa DeShivs post haste wanting to talk to me about the altered serial number. I explained what happened, showed them the paperwork from the FFL where I purchased. They asked me to sign a statement attesting to this. I did, and that was the end of it. They told me then it was perfectly legal for the PD to do this.