You cannot register an existing NFA firearm. The Form 1 is for building a new NFA firearm and that firearm can only be "manufactured" after the approval of that Form 1. Only the amnesty of 1968 allowed existing NFA firearms and devices to be registered. This also applies to FFL SOTs, they cannot register existing NFA guns either. It's the same for all NFA weapons, including MGs.
Possessing it is a felony. Trying to register it is a felony, perjuring yourself by lying on the forms is another felony. If you sell the resulting firearm ever, that's another felony. Then multply that by the number of Auto Burglars you registered. It's a very big deal.
Auto Burglars are very well documented firearms and a serial number check of the weapons or any research into older Ithaca records would likely come back showing that they were manufacturered in the configuration you registered them in, meaning it would prove you didn't "manufacture" them as NFA weapons, they already were NFA weapons. An original barrel on an original receiver with no amnesty paperwork or 1968 registration date would also be an obvious sign that there is something not right.
Even if you could outlast the statute or limitations on the registration, the guns are always going to be contraband and subject to confiscation at any time. Selling the contraband firearms creates a new statute of limitation from the date of the sale. A new owner would then be subject to having the guns taken also.
Tough situation, but not much to do legally.