From Stamps of the World
Cambridge is a university city and the county town of Cambridgeshire, England, located on the River Cam about 50 miles (80 km) north of London.
Cambridge was allocated the PO Numeral 158 in the Post Office lists of 1844.
Hills Road 
Hills Rd office was an early receiving house of mail, mail would be handed in for posting but not cancelled, instead the receiving house would backstamp the letters and send on to the main office in Cambridge for cancellation and checking before being sent to its destination.