I read about half of this.
First, the anxiousness I hear about the stock and the pistol is always a bit funny. It IS true you can get into trouble putting a 4 inch barrel to an artillery stock. I can't see how anyone would get into trouble owning both, and not owning an artillery luger. Come on folks, give a warning that a 4 inch is not legal - although to be honest, if you read the various BATF luger and stock combinations it is pretty crazy.
SO, DON'T PUT A STOCK ONTO A 4 INCH (OR 3/58) LUGER
However what is confusing is that an;
Artillery Luger is legal with an artillery stock
Navy is legal with a Navy stock
A Finnish Luger is legal with a stock (this is where it is not very precise)
In addition, an Ideal stock can be fitted to an American eagle Luger or a Breinkee (mispelled I know)
Carbine stocks can be put onto carbines
So, cut the guy a break 
for owning a 4 inch luger and an artillery stock, when his Grandfather shot it, it was legal...