"CASTLE CARY S.O / SOMERSET" cancel from 1921

Castle Cary became a Railway Sub-Office in 1898, and then a Sub-Office / Post Town on 1^st August 1905 when the Post Office stopped using the R.S.O. terminology.  

In 1920 it came back under Bath, having previously been under Bath before becoming an RSO in 1898.

The “CASTLE·CARY. S.O / SOMERSET” double circle double-arc cancel was previously known from October 1913 to September 1918.

The postcard below extends the known usage to October 1921, from after when Castle Cary moved back under Bath, and when it should really have been "CASTLE CARY / BATH".