This page details the intricacies of the Thornton tandem axle conversion (which apparently sold well into the 50's). From this description, it appears the Thornton locker was actually used between the drive to the middle and rear differentials, and not within an axle differential between left and right axle shafts. However, the depiction of the locker is precisely the same as modern day units sold as "Detroit NoSpin" lockers, which install within the differential and replace the pinion or spider gears. It is not technically impossible that this centre was fitted within the rear differential in the 4x4 FGT.