A constructive comment here to assist you to research your family ........you need to be curious.
Don't you find it interesting that Cecil MARTIN dies as MARTIN and COCKBURN.
Newspaper notice names him as COCKBURN only.
He must have been known as COCKBURN.
SMH 12 Nov 1947
http://trove.nla.gov.au/ndp/del/article/18034715?
........beloved brother of M COCKBURN, privately cremated 3 July.
Who is M COCKBURN?. From the index entry, when Cecil dies there is someone around who knows -
...that COCKBURN is not his real name.
....can supply all three given names for him....he is not just Cecil MARTIN aka COCKBURN.
...can name his parents.
Start by looking at electoral roll 1936...... at 54 Avenue Rd Mosman
Mary Ann Elizabeth COCKBURN
and other COCKBURNs in that electorate.
I suspect that Cecil has not just assumed another name, but seems to have attached himself to the COCKBURN family, and I would be wondering why he would do that, and why that family, because I cannot see a connection between the two families.
I would be getting that death certificate to see who the informant is, and what other information is on the certificate relating to marriages, spouses, issue.
N S MARTIN, the son b. 1919 Cairo...I would be thinking that "S" is likely Sidley / Sydney. Cecil made no effort to hide his name. Cecil is in Cairo in 1918 - 1919, and returns to Australia in 1919 from Cairo.
Why did the mother of the child, Miss WIDNER, an Austrian national, not look for Cecil via the Australian Army earlier than 1946 to at least resolve her marital status, so that she could marry again?. Enquiries about such domestic matters happened a lot after the war and were handled by the government, the better to resolve claims for gratuities, pensions, ongoing commitments to dependents etc.
I am not suggesting that you actually follow up on this, it is all too long ago, but I would be thinking there is more going on here.
And I would start with that death certificate.