Perhaps someone in the UK had not heard that William was deceased when the letter was written, and 'John' was from a different family?
Bernard would not have had a son John in his twenties by 1943. ("It is information about John in his twenties being written to William")
NSW Marriage Index 5259/1934ANNANDALE HALLBernard &MCMAHONKathleen M
Bernard was born in the UK according to WW2 records. HALL, BERNARD Service Australian Army Service Number NX200439 Date of Birth 5 Apr 1910 Place of Birth ENGLAND Date of Enlistment 2 Apr 1943 Locality on Enlistment PETERSHAM, NSW Place of Enlistment PADDINGTON, NSW Next of Kin HALL, KATHLEEN Date of Discharge 1 Apr 1946 Rank Driver Posting at Discharge 2/33 W/S PL
UK freebmds Marriages Jun Q 1909 HALLWilliamOldham8d 1020 MOOREEdaOldham8d 1020