A family tractor must be the best one to get for restoration. A good guess is that your Grandfather's is either a B or BN, although neither has an offset rear axle. It may have seemed so because of an off-center seat. They both had a narrow front end. The only '40s vintage Farmalls with offset rear axles were the A, Super A, and Cub. All 3 had wide fronts.
I hope you are able to get it and preserve it for future generations.