searching for info on tumblers I have that has a square in the bottom with intial J - have never seen this myself.  anyone have a clue?

Can anyone out there tell me if there is a good Northwood reference book on their items besides Carnival Glass.  Northwood made so many more items than carnival and it seems there is nothing out there but that on reference.

I have a new custard piece going up for auction soon and need to find out as much about it as possible.