Hi Sue,
That new feature that NSW BDM provided recently has come in handy ! Gweneth L FORDHAM date of birth is either 9th June 1915 or 10th June 1915. So, as today is 11th June 1915, the NSW BDM online index has only just in the past day, released the information (under the 100 years rule). (Add, Sue's post with the births for the Fordham couple was uploaded 8th June, so just a couple of days before the 100 years had expired before Gweneth's bc reference was listed)
Gweneth L, daughter of Arthur G and Lena A FORDHAM, registered Waratah District with ref no. #46255. (I have not found her twin 
and I tried to find the births online on the 10th June (ie yesterday as I write this up) but not there, so perhaps Gweneth was born 10th June.
I also tried using "Waratah" as the district, and the dates to search 9 June 1915 to 10 June 1915, and the wild card for the surname. So, perhaps Gweneth was the elder of the twins, born on different days.
Cheers, JM
As another day into June has come on to the online search facility (11th June 1915) I have searched again for Sylvia on that online search facility and have had no success.
I have used just the registration number and again no result.
However, it is clearly an error and, overall, unimportant as you have from my other source the information, and it is without doubt, correct.
FORDHAM, Sylvia L.
Father Arthur G.
Mother Lena A.
Waratah. 1915
Reg. 216254.
As to the shift of birthday celebration to place their conception inside the marriage, I think we would find it quite a common practice.
It seems likely to me the twins made this move themselves at some point in their lives rather than it being an earlier parental decision. A six months leap would be hard to explain in childhood development
I am not at all familiar with military uniforms.
On Rootschat there is a board Military Forces. You could post the photo there and ask for help.
The Australian Board also has chatters who are knowledgeable in the field. That also would be worthwhile I think.
Sue
