There's an interesting looking marriage in 1935 in Cardiff, for a Charles H H Harding and Agnes Bolson, son Brian Anthony born 1939 (mmn Bolson, died 2019), and 1939 register, still in Cardiff, where he is simply Charles Harding, but .... date of birth agrees with the 5 Nov 1902 school entry found by sgf28, and he was a E N G Apprentice Proud & Son Docks Cardiff in 1921 and a Yard Ship Fitter Heavy Worker in 1939.
Death on the GRO of Charles Henry H Harding, Q3 1991 in South Glamorgan (includes Cardiff), born 1905, and a burial for a Charles Henry Hubert Harding in Pantmawr cemetery in Cardiff in July 1991, find a grave says birth 1906 so slightly out, same plot as Agnes Harding, died Nov 1921.
Only possible obviously, but might explain why there are no records for Hubert; the marriage / death certificates would hopefully confirm (or deny).