39 Class 104 powercars (50420-428/430-443/480-494/529) were built with a single roof mounted marker light on the front end. This, plus the 2-character route indicator, provided the only forward illumination. It was felt that when the route indicators were being removed in the 1970's that the single roof light would not be sufficient, so many of the 40 vehicles with these were fitted with black covers over the old route indicators with two white circles for the bulbs to shine through, maintaining the same forward illumination as before. These were known as "Domino" headcodes due to their visual similarity to the pieces used in the game dominos.
Vehicles confirmed as having had Domino headcodes include 50420/425*/427/428/434-436/484-486/488/491/494 but there are likely to have been more. The earliest sighting of one fitted was in April 1977 with the last known sighting being June 1985. (*50425 was fitted with a non standard version where the normally black areas were yellow).
From 1981 as vehicles went through works, the Domino headcodes were removed and an additional two marker lights fitted in their place, these were located in the normal position for later built Class 104 vehicles. The original roof mounted marker lights were either left dormant at this point or removed and plated over.