TPO had ridden it with the g/box pivot hanging out, thus odd wear to chain and sprockets. G/box sprocket issue. The one fitted has splines and flutes of the same dimmensions, and the replacement has narrow flutes, about 1/2 the width of the splines. So the question is.....any ideas as to what machine the gearbox orriginaly would have been fitted to. It is marked STD.
I think only gearboxes for the swinging arm BSAs were marked as STD. GoldStars were RRT2 for instance.
The flutes/splines on the sleeve gear of the swining arm heavyweight gearboxes are all the same.
They were available with different number of teeth and the B31/B33 had a narrower sprocket to take a 5/8 x 1/4 rear chain til 1957. The A7/A10 and 1958 on B31/B33 took a 5/8 x 3/8 chain with wider sprocket to suit.
Photo shows the sleeve gear flutes/splines - left from a box STD T with layshaft needle rollers and right a box STD with layshaft bushes - the same flutes/splines on both.