Peter,
My understanding of the pre/post-86 definition is that 1986 was the year after which no additional fully transferable machine guns were to be licensed. Therefore the classification, as it pertains to foreign manufactured firearms, is not a function of when it was produced but, rather, when it was imported. Post-86 MGs are supposed to remain in the hands of dealers, law enforcement and the government only.
Again, this is my understanding so, guys, if I've got any portion of this wrong, please feel free to correct me.
Cheers!