Searching for something else I found this link and thought I can add to the story as it is now the subject of a book - The Kings Private Army: Protecting the British Royal Family during the Second World War by Andrew Stewart.
It appears six Ironside special armoured staff cars were initially build as shown in the photo above and layer two Type B with additional side armoured windows as shown in this link
https://www.keymilitary.com/article/protection-detail
Based on Mk 1 Humber LRC they feature enclosed and raised roof with ventilation system, wider rear hull sides, full length doors and a plush interior.
Paul