TyneMouth Borough Police were separate from the old Northumberland Constabulary, until 1968, when Tynemouth Borough lost their independence. !and they were merged with Northumberland. I do not know when Tynemouth was formed. Another independent force was the River Tyne Police, and they too were assimilated in 1968, as was Tynemouth Borough. All the old North Eastern forces, including those south of the Tyne had a minimum height requirement of 5'10" to qualify for acceptance.
Search the web for 'British Police History' and you will find dozens of entries re old 'defunct' forces, and current forces, together with many old photos therein.
Best of luck in your search and if you find what I have suggested you may well find much info. re Tynemouth Boro'.