I have a Smiths gearbox here surplus to requirement which came with one of my bikes and is bronze body. It was in a box of NOS parts and certainly doesn't look used and feels really smooth when rotated by hand. 2:1 ratio.
Look at the body shape that is slightly different to the one Chris posted, but agrees with the MO sketches and the diagram in Know Thy Beast.
Pat number on it is X70549/1 which agrees with Smiths list for Black Lightning fitting.
Simon