I believe that the finish should be the lacquered finish. I have achieved a good reproduction of this finish on my 1930 2 Speeder. The first requirement is to obtain a near polished finish on the cast iron by using ever finer grades of Wet & Dry Paper ending up with 1200 grade. The block is then ready for lacquering.
Buy a tube of Alizarin Crimson from an oil paint supplies shop, and mix a few drops of this with some clear polyurethane varnish. This will give a crimson lacquer. You will need to experiment on a sample surface to determine how much to add to get the desired finish. I gave my block a couple of coats. The finish should last about 5 to 10 years but some rusting can start under the lacquer near the water dome.