It is two numbered corrections, made whilst the register entry was being completed.
You can see that the father's name has been crossed through (the first correction), and then the word "formerly" under the mother's name has been crossed through (the second correction).
That would suggest that the registrar was under the impression the birth was to a married couple, until at some point during the completion of the entry it became clear that wasn't the case.
Although this isn't an image of the original register, corrections made are reproduced on copies submitted to GRO, and so appear on any certificates/PDFs they provide.