A deactivated DD does not need Dewatt paperwork. They are sold as title 1 guns just like a remington rifle. You have to form 1 it or a manufacturer form 2 it to reactivate it. Also the gun must be stamped with the new makers information.
I would not let a novice gunsmith work on one. Find someone that has experience with them. There is a lot of energy in there and you can get hurt quick.
Under the ATF website however, it says that manufacturing DD's takes a class 3 title 9 or 10 license, that a regular class 3 title 2 manufacturer of NFA weapons does not include making destructive devices...
So are you saying that I can transfer a Lahti without dewat paperwork that is deactivated to a regular FFL like a regular rifle, and to reactivate it I could do so with a form 2? (what's a form 2 exactly?)
This doesn't sound right because legally it's either an illegal DD to begin with because the receiver would already be intact and premade, and it's basically just a rebarrel job. If it didn't have the dewat paperwork you'd be essentially manufacturing a DD out of an illegally dewat'ed DD... right?