The term refers to a Class III weapon manufactured after the 1986 McClure-Volkmer "Firearms Owners Protection Act", which banned the continued manufacture of Class III weapons for civilian sale.
Thus, "civilians" can own pre-1986 Class III weapons. Post-86's are dealer samples (for showing law enforcement, for example, who may be interested in purchasing) or for police/military use.
We, the serfs, cannot legally own them unless a special class of firearm manufacturer or dealer.
I may be wrong on some of this, but not by much.
TimW
Phoenix