WilliamT- I wonder if that guy who brought in the MP40 could have had his uncle's deactivated war trophy? Probably not, since you did not mention it was deactivated. BUT, if it was a deactivated war trophy and the Form 6 Treasury Department paperwork was found, it could transfered as a dewat then reactivated to a machinegun. Following NFA and BATFE rules, of course.
I guess my point is: be on the look-out for DEWATs w/ papers, they are valuable and need to be saved.