Not an OEM finish as far as I know. Probably a previous owner's attempt to make the dull grey more attractive without a lot of intricate polishing.
I've found a light coat of "chrome finish" enamel with a coat of high temperature clear engine lacquer to be fairly durable and look quite effective on aluminium castings, but I think on a carb it would be very dependant on the petrol resisting properties of the lacquer.
I intend to give the two carbs on a Daimler car I own a tidy up using this method, but with there being no tickler involved in the start-up process it would be a bad day if I had petrol running down the body.....