I think the "R" on the index for Ada's birth is the revision of Margaret's birth surname on the document
There is no revision of Margaret's birth surname on the document. The revision is the crossing out of the details of Margaret's marriage to Moore.
But
R in the Victorian Indexes does not stand for Revision. For example I am looking right now at a certificate for Agnes Louisa Johnson born 5 May 1893 in Bendigo Victoria. The certificate was revised in April 1894 after a court case established that Agnes Louisa's mother Louisa Johnstone was not married to the man that she had said was Agnes Louisa's father. The revision crossed out the name and details of the father, the details that had been given of a marriage (the details were ficticious), and the "Clingam formerly" in the middle of "Louisa Clingham formerly Johnstone". This is a distinct revision, but the index number is 10062, not 10062R.
R in the Victorian indexes stands for Repeat, and is used for when a child is born illegitimately to a woman who is either married or widowed. The birth is shown twice on the index, as in Ada's case once under the mother's married surname, and the second time under the mother's maiden surname. The surname of the child is not shown on the certificate, and as this child is illegitimate (in this case with no father named), she is shown twice on the index under both the surnames that her mother has used. Another example is Fanny Gladys Hughan/Campbell who was born on 2 Nov 1904. As her mother is shown as Elizabeth Hughan formerly Campbell, and as Fanny was born illegitimately, her birth is shown twice on the index, once under Hughan and once under Campbell. (In this case the father was named, George Walter Butler, and he even registered the birth, but his surname is not used, and he is not shown as the father in the index.) The index for Fanny's birth uses the number 27481R. There is no revision of any type on her birth registration.
The R is used ONLY to the index. If you go onto the BDM VIC web-site and try to order a copy of the certificate and include the R you will get an error that the index number does not exist.