It's a very fine tricky line that can be crossed only by BATF .
Not registered now currently but a gun that was made before 1968 and is on amnesty list may still poss be able to be granted legal status .
One of those who you are , what it is , who is thinking about it and
Up until 1968 folks who had unregistered guns could come forward and reg them. Many did many did not .
Some came forward a couple yrs later and were granted legal status .
But in mid 70-71 the Supreme court sealed it and set a point in time where going forward you could not reg the gun and were now in possession of an illegal MG which carried ten years felony charge. Maybe the owner simply panicked and chucked it ?
Gun may have been in a museum and stolen , too big and the thieves shortly realized they had no place for it so into the creek it went .
I don't think it was used in a drive by
G.I bringback and at 80 he got tired of it in his closet ?
Safely disposed of it in the 'crick' !