From Nanny Jan's post for Harry Davidson ORR death record that would make his birth year around 1880
You can search the 1911 census for free but it cost credits to view, this should confirm his place of birth and occupation as the 2/6th Battalion formed at Chesterfield on 14 September 1914 as a second line unit
http://www.1911census.co.uk/search/tnaform.aspx?&x=1000195671901c living DerbyshireHenry D ORR bn 1880 Scotland occupation Mining Engineer Pupil
1891 census SussexHarry Davidson ORR scholar 10 bn 1881
this is the only census I can see with his middle name Davidson perhaps someone would kindly give you the full transcript and can check his place of birth
There is this family in Scotland which may be Harry with his parents

1881 living Renfrewshire ScotlandRobert ORR bn 1829
Mary M Orr bn 1839
Francis M
Mary G
Mabel A
George W 1875
Violet H 1877
Harry D Orr 1881 IGI extracted marriageRobert
ORRMary M
ADAM10/9/1862 Milton, Glasgow, Lanark
Again the original should be checked as there's this entry on
IGI family pilot search christening
Harry Alexander ORR 12/12/1880 Neilston his parents are named as Robert ORR & Mary Mcindoe ADAM – the initial D could be a transcript error on 1881 census and this is not your Harry Davidson ORR
There are however these
extracted IGI christenings Neilston Renfrew, Scotland
parents named
Robert ORR & Mary Mcindoe ADAM
Robert Charles 26/7/1863
James Graham Adam 10/7/1864
John 28/8/1865
Charles Edward 21/11/1866
Francis Maxwell 31/5/1868
Archibald Ewing 31/5/1868
Mary Gertrude 26/12/1869
Mabel Agnes 4/05/1872