The MDB is a "one piece" magdyno isn't it? It's all aluminium and normally they are unpainted. Some military (where applicable) ones might have been painted but not that period.
Some American magnetos are painted all over( Dixie's and aeros) With the normal Lucas up to about 1930 with exterior magnet the magnet is enamelled black (semi flat usually I stove them and then remagnetise). The brass parts are dull nickelled. The aluminium is not polished. Some parts of the surface appear to be belt sanded /linished on the "monobloc" magdyno which went till about 1932. They are a very neat looking unit but delicate and hard to find/or build, good working ones. Quite a valuable thing to have on a good example of a period machine. Nev