I have no idea how it will work going forward, but the ATF doesn’t care what serial a home builder assigns. I’ve scratch built SBRs and I needed to serialize it to complete the Form 1. It doesn’t matter if the serial conflicts with any commercial guns, because I’m the manufacturer. What Mass wants (i think) is for FFLs to serialize anything that they take possession of. In that case, they would need to make sure pre ‘68 unserialized guns aren’t given a serial that conflicts with any post ‘68 versions of the same model. I thought I read that FFLs would be assigned their own serial prefix to cut down on accidental duplicates.