Thanks for all of the info (well most of ya'll anyways). I guess my whole reasoning for asking should have been more clear....I'm not just wanting to bait the ATF for this. My grandfather is a machinegun dealer and has M-16s (pre- and post-'86, papered, etc) and I wanted to get him this for the one that doesn't have a 3 rd burst for Christmas (really easy gift for him). But since he lives in New Mexico, I would probably take it when I visit him around the holidays so it would have been in my possession for a few months, hence me asking.
Thanks again for the info esp. clearing up the Drop In Auto Sear and Lightning Link misunderstanding as I'm not an AR/M-16 platform guy.
If it were me I would not worry about it then...you don't have an AR-15 - even if you did it would need to have new pin holes drilled and machining to create an illegal machine gun. I would get it for gramps and not worry then.