Yes both birth reg and baptism. No other Arthur born to the couple after marriage. No Arthur Herberts born 1891-3 with mother's maiden name Castell or in the right area. None on the census(. I've no doubt this is the same man. )Even at his marriage in 1916, he gave his correct age of 30.
The change of name is understandable not so much the age. In 1921, with his age correctly written he was living next door to his uncle.