That looks like a battery carrier from an M20 and I'm not sure whether it is the same as a B31 but I'm sure Julian will come out with the answer shortly, if it is the same as the M20 then there is a plate that fit behind what you have, secured to the frame with three studs and the acual carrier is clamped at the big hole, so that you can adjust the angle correctly. HTH.
TTJohn