Well.
Assuming that William's age at death is correct, he would have been born in 1865. The IGI at
www.familysearch.org has an almost 100% complete index of births in Scotland from 1855 to 1874, so I would expect to find a birth listing for him there.
The IGI lists just two children to parents of those names: Grizal, born 1763 and Elizabeth, born 1764, so these are obviously not yours. It does not list any Alexander Duncan whose mother was a Montgomery.
Nor is there a marriage of a William Duncan to an Eliza Montgomery, though a William Duncan married a Mary Montgomery in Rutherglen in 1862 and had six of a family, not including an Alexander.
I wondered if he might have been illegitimate, but the IGI has only one listing for an Alexander Montgomery, mother Eliza Montgomery, born 1 June 1865, and this is a 'submitted' entry with no exactly matching 'extracted' entry, therefore its accuracy is suspect. (He is probably the son of Robert Montgomery and Elizabeth McEwen, born 1 July 1865 in Ayr.)
There are five Alexander Duncans with father William born in Angus between 1860 and 1870, with mothers' surnames Sheriff, Lindsay, Laird, Steel and Ormond.
There is no Alexander with mother Montgomery born in Forfar between 1860 and 1870, and though there are three Alexander Duncans, their mothers' surnames are Duncan, Sinclair and Ormond (the same one as above).
It is just possible (but very unlikely) that your Alexander is missing from the IGI, in which case he would be listed in Scotland's People at
www.scotlandspeople.gov.uk but it would be quite a job to cross-check all the births of Alexander Duncans to eliminate those in the IGI against those in SP.
I see that he had been in NSW for 42 years, suggesting that he arrived there in 1882, when he was only 17. This seems a bit on the young side to emigrate alone, so I wonder if you have searched the lists of arriving passengers to see whether he arrived with other family members?
I wonder whether 'Forfar' on his marriage certificate means the
parish of Forfar or the
county of Forfar (Angus)?
I had a look in the 1881 census of Angus for any Alexander Duncan who might be yours.
There is a 19-year-old factory labourer Alexander Duncan, born Dundee, living in the household of his sister Elizabeth at 15 Patons Lane, Dundee, and an 18 year-old house painter, born Montrose, living with his younger brother John in the household of their aunt Margaret Duncan at 21 Annfield Road in Dundee. Alexander Duncan, aged 18, brn Kirriemuir, was a farm servant in Glenmuick in Aberdeenshire. Another 18-year-old Alexander Duncan, born in Maryton, was living at 69 North Wellington Street, Dundee, with the family of his elder brother David. The only Alexander Duncan of roughly the right age and with a father named William is the one whose mother was a Sheriff.
So I am stumped for the moment.
