Obviously, one is mainly brass, and the other mainly
steel. Points adjustment on steel ones is 'car-like'. The steel version was said
by Lucas to have a contact breaker lever with reduced inertia and to allow
higher speed of rotation.
It is important to know though, if doing serious work
on the steel sets, that the MOVING point is LIVE (ie attached to the LT coil),
whereas on brass backplate versions the FIXED point is live.
Further information is contained in a supplement to
the Lucas Workshop Instructions, for which