Date on Glos BMD site is 1917 whereas it is 1918 on other sites:

FreeBMD:
Births Mar 1918
Chamberlain Sidney B Chamberlain Westbury S. 6a 382

GRO:
CHAMBERLAIN, SIDNEY BRUCE -
GRO Reference: 1918 M Quarter in WESTBURY ON SEVERN Volume 06A Page 382

I suspect he was born late 1917 and registered in 1918.

Mike