Pre/Post may? I've heard of post '86 but I don't know anything about May.
Elaborate por favor?
The Firearm Owners Protection Act of 1986 (FOPA) was what instituted the cut off for new civilian manufacture machine guns, as of May 19, 1986. After that date, no new machine guns could be made that were for civilian manufacture. Thus, ALL new MGs after that date were "Post samples" or "Post May Samples", which means that a Class III SOT/Licensee could have one, but only if they had a demonstration letter from a law enforcement, military or other qualified agency, indicating that said agency/group wanted to evaluate Gun X, and that Dealer Y was the one going to do it for them, therefore having Gun X was necessary for Dealer Y.
Back up to the Gun Control Act of 1968 (GCA). This law prohibited further importation of foreign machine guns except as dealer samples, which mean (I believe) they needed a demonstration letter for these. This is how many of the factory IMI Uzi machine guns (that were originally FA, and not converted) were brought into the country. Since these are dealer samples, but were before the end of new MG manufacture, they are considered Pre-May Samples, and can only be purchased by Class III SOT/Licensees, however they do not need a demo letter and the dealer (if licensee is an individual) or company (if a corporation) can keep them if/when they give up their Class III FFL.
That's how we got Post-May Samples/Post-Samples and Pre-May Samples. Of course, prior to May 19, 1986, one could legally convert any semi-auto in the US if you got the tax stamp. So, a lot of folks got IMI Uzi Carbines, filed the paperwork, then converted either the bolt or the receiver to make their MG. AR-15 folks could drill a hole, etc.