There has been some discussion about this on the Banjohangout forum.
One thing for sure is that you would need to know more than the seller is showing to be able to verify if that is a prewar Gibson. It is almost surely NOT a prewar Gibson as the seller would show signs of proof if this were so as a prewar one piece flange conversion (that is, a tenor with a 5 string neck) would be worth upwards of $4000 and anyone trying to sell such a piece would go out of their way to prove its originality.
There are so many fake Gibson necks out there (Gibson seems to have had nor presently seems to have any concern about anybody slapping a Gibson logo on a banjo neck) that you need a pretty high level of knowledge if you’re trying to buy any used Gibson banjo from an unknown source.
The neck might be Gibson but positively not from the 30’s. Without seeing the the inside of the rim and real closeups of the flange there is no way of knowing if the pot is even a Gibson.