I had been studying the history of Hertfordshire police cars for many years and was not aware that the force had ever used Rileys. However, in 2015 I was sent some photos by Jean Campkin, the widow of the late PC Ron Petts showing Ron with some police cars. One of them shows a Riley RM model although few other details are known. One of the photos shows Ron Petts standing by a Riley. The front of the image is blurred, but there is a second photo with another officer, called Frank ??? (maybe Frank Allen), which better shows the front of the car with a police bell fitted. It is not possible to read the registration number clearly but it looks like ?RO ???. The original prints were tiny, 2″ square, and I was unable to obtain any better scans from them.
I asked if the Historical Society had any other photos of Riley police cars and the only one they could find was a brief view of the boot of one register KAR 83 which is inside a dark garage.