Not found a birth for a Stanley Gosling so now I am thinking idiotically I know but how about this:-
What if Stanley was born a Fieldsend and as he is much older then Avis is the reason he is not on a census with her and could he in fact be sending the card to her as he boarded the ship back to England to let her know he was on his was back. He just put Mrs Gosling by mistake when he should have put Mrs Stanley.