Since posting my AB air reservoir question, I've reread Gene Green's paper on freight car brakes. A study of the photos and diagrams in this paper suggest that the AB air reservoir has an up and down side, with the 'fat' side hanging down from the floor mounts, and it is usually mounted on the opposite side of the the center sill from the brake cylinder with two lugs facing the B end (direction to which the brake cylinder points).
Applying that convention, when the air reservoir is mounted on the opposite side of the center sill from the brake cylinder, two lugs face the B end. When the air reservoir is mounted on the same side of the center sill as the brake cylinder, one lug faces the B end. Cars converted from KC to AB brakes frequently have the AB air reservoir on the same side of the center sill as the brake cylinder hence the FGE car has one lug facing the B end. I think that pretty well sums up the answer to my initial question.
