In the start of the 70 production bsa fitted a shorter torque
stay and mounted the arm the other way up to give a more effective break, sadly the short arm had a tendency for whipping up (presumably moving the wheel back also???) and causing the brake to lock on. They reverted Back to the earlier style with the longer torque
stay and down mounted lever.
If you wanted to get fancy you could change the internals, fit an oif cam, match it with the arm and have a fully floating SLS rear brake which will be much better than the pre oif models yet looking almost exactly the same.
With this some levers and cams are for downward use and some upward, notified by a square section for downward arms and square with rounded at 2 sides for upward arms. This replaces the spline on earlier pre oif models.