Sorry if I confused the issue Ed, I was really only mentioning that the Met. had a similar insignia. The rank was for a senior sergeant to be in charge of the front office in a busy police station. They abolished the rank - I think, in 1972 by promoting all S/sgt's to the rank of Inspector.
The book is out of print - although copies are always available on Google at about the US$40. I have a number of mint copies and offered through PICA (Police Insignia Collectors Assoc.) a special offer of £14 incl. of p&p in the UK. Quite a number were ordered as there are only three major books on Police Collecting. For Google - look for 'The Policeman's Lot - Quiller Press. 1985. Mervyn Mitton. If anyone wants one direct IM and I will give you a UK address who will send by post. (Sorry for the commercial - but , you did ask.)