Shttp://www.armslist.com/posts/240369/louisville-kentucky-rifles-for-sale-trade--m16-upper--no-bolt-or-bolt-housing--20- Saw this on Armslist, and thought...wth? Sorry thought I could put the link on here....
The bolt carrier for an M16 is different, but it's not included, so there's actually no way to distinguish the upper from AR15. Any upper could potentially be called "M16". Probably just a marketing ploy.