I may well be incorrect and that's nothing new however I always understood that the R suffix to the registration number indicated there were two registrations ie the registration was repeated with the same registration number as there was either conflicting information or the details were not clear. Another reason springs to mind....if the couple marry after the birth is registered and if the father wishes to have his name noted on the birth as such. When researching in Vic I have also found births registered years after the event and the letter R as a suffix. I do have some literature 'somewhere'.
I have to agree. I have copies of VIC birth registrations that have an R on the index but absolutely no revisions on the certificate. They instead are registrations of illegitimate births and have at least 2 inclusions of the registration on the index under at least 2 different surnames (I have never seen one with more than 2 surnames, but you never know). When you order the certificate from BDM VIC the R is not used. If you try to use the R when you order the certificate you will get an error. The R exists only on the index.
It begs the questions 
What was the impediment to the first marriage?
If there was no first marriage
What was the impediment to the second marriage?
Sue
I agree Sue. It also begs the question of why would Margaret make up a marriage for the registration of the birth of a baby to another man?
Mind you, as far as I am aware, at least in NSW, there was no requirement for anyone to "prove" they were a married couple when registering a birth (I guess regulations in VIC in 19thC would be similar) ....
I don't know how someone would have proved whether they were married on not, but the question was definately asked of the informant when they appeared to register the birth in VIC. The registrar was looking for the mother to be married to the father, and not to someone else, otherwise they often wrote the word "illegitimate" on the document. Perhaps people were sometimes too honest and didn't think to lie about their marital status? You would think that if Margaret was going to make up a marriage for this registration it would have been a marriage to Sydney Woodland, and not one to Michael Moore.
BTW I have just looked at Cornelius's birth registration - it says that the marriage to Michael Moore was on the 8th January 1879 (as on Ada's registration) but this time that the marriage was at Cranbourne. This just confirms that Sherwood was next to Cranbourne and would form part of Cranbourne today. Margaret was 17 years old (she had just tuned 17 when Cornelius was born, and it still says that she was born at Sherwood. The informant was the father Michael. Again the registration says "after declaration made by law as required, with a date of registration of 8 May 1880, and a date of birth 17 January 1880, which just confirms what others have been saying about this meaning that the birth was registered late.
Unfortunately just because a marriage does not appear in the registrations does not mean that it did not occur, just that the minister forgot to forward the details. The only way to find the original record of the marriage is in the parish register of the church in which the marriage occured. Bummer not knowing what was the denomination.