Maybe fitted after leaving the assembly line. Why customise individual cars on a production line where time and motion study was the norm for Ford management. I know this from experience.
Given such a variety of non-British OEM makers parts used, such as Marchal Citroen, DKW, and Volvo et al, reflecting the car market at the time, it would appear that the buyers were using after sales Brown's Brothers accessories catalogues or similar.
Another question for someone - when and where were these Lucas lights first used ? P/1049 has them.