If Mary took the surname of her employersat the age of 16 (John & 1st Mrs Rees) then a possible marriage could be:
John DAVIES, Mary Louisa REES Sep 1879 Carmarthen 11a 863
Death of hubby could be 3 yrs later:
John DAVIES 20 Sep 1882 Carmarthen
with birth of Daur:
Agnes Mary DAVIES Mar 1881 Carmarthen 11a 813
nicely sandwiched inbetween
John DAVIES, Mary Louisa REES Sep 1879 Carmarthen 11a 863
Death of hubby could be 3 yrs later:
John DAVIES 20 Sep 1882 Carmarthen
with birth of Daur:
Agnes Mary DAVIES Mar 1881 Carmarthen 11a 813
nicely sandwiched inbetween