Supposedly it had to be built into a complete rifle before the ban in order to be grandfathered in. This ban covers semiautomatic rifles and only semiautomatic rifles can be grandfathered in. The ban did not affect lower receivers per se, so a lower on it's own would not have been grandfathered in.
The relevance? EA made lowers and either didn't make rifles or didn't turn all receivers into rifles. Hopefully in the time that passed before their demise and the passing of the 1994 AWB, they were all sold as kits and built into rifles. The guy who had a crate of unbuilt kits in his basement is the one who would have lost out-
I am pretty sure that the burden of proof is on the government, but in CT you might have to be the one to prove that it WAS versus the state proving it WAS NOT.