The importation law of 1981 banned all foreign produced MG's to be imported for civilian use (read form 4). From 81 to 86 only American made MG's were added to the books for form 4 transfers. If the imported MG was not on the NFA books before the importation ban of 81, it cannot be sold for transfer to civilians (form 4).
Some MG's "post 81/pre 86” MG’s (imported for LEO sales, retained by the dealer on a letter of demonstration) where on the books as dealer samples for LEO transfers, and retained by the dealer after giving up there FFL's (became their personal collection), but these MG's can’t be transferred to others civilians for a form 4 transfers, and can not be passed on as inheritances at their death.
If the MG (USA or foreign) was not on the books before may of 86, then it can’t be retained by a retiring FFL dealer, and must be sold off before given up his FFL.
Bottom line is even if the ban of 86 is voided, the importation ban of 81 would also need to void to allow a form 4 transfer of an imported HK transfer for civilian use.