Burnham was made a Railway Sub-Office (R.S.O.) on the 1st December 1887. This lasted until August 1905 when the terminology in the Post Office changed and it became a Sub Office / Post Town (S.O.).
The double circle double-arc RSO cancel was in use beyond 1905 and then the cancel was altered to remove the "R." of "R.S.O.".
On the 2nd August 1917 Burnham became Burnham-on-Sea.