Here is the answer as I understand it.
If the DATE of the manufacture is prior to the cutoff, but it is an otherwise "stock" mini-14, you don't have a preban.
A preban assault weapon is a firearm that was in "assault weapon" configuration according to the Fed's rules the day the law went into effect.
In other words, if you modify a stock Mini-14 to have pre-ban features, you are manufacturing a new assault weapon, after the ban, and therefor are guilty of something or other.
If the owner on the cutoff date had the rifle in an assault weapon configuration, then it is considered a grandfathered assault weapon.
Thats the KEY word... GRANDFATHERED. You cannot change a non-assault weapon configuration firearm into an assault-weapon configuration legally after the cutoff date.