I actually called the ATF quite a few times about this. From what I was told, the ATF has some records from Olympic because they went to the factory right after the ban. But even they don't have a complete list.
I was concerned about this as well, so I got a letter from the guy who bought it from a guy who had it as a rifle since the 80's. Good enough for me.
See, it's not that it had to be shipped as a rifle, just that it had to be built into a rifle prior to the 94 ban.
In other words, all that had to happen to it prior the ban is that a upper receiver had to be slapped on it before the ban.
Say what you want about the burden being up to you to prove it was assembled prior to the ban.
In reality, they would need some kind of proof that it wasn't to arrest you to begin with. Other wise they could just arrest everyone with a preban (including Bushmaster and Colt) they came across, forcing them to get lawyers to "prove" it was a preban in court via the serial numbers.
And I have yet to see any evidence f anyone even slightly fucked with by the ATF in relation to a "questionable" Olympic lower.
And there are THOUSANDS of people selling, and possesing them.
Many of them on this board (me included).
Ultimately it's your call. But if manufactured in the 80's, I wouldn't worry about it.