My great grandmother was born in January 1905, but her birth was not registered until June quarter (i.e. April or later). Her son's birth in the 1920s was registered 57 days after his birth.
Births in England and Wales were supposed to be registered within 42 days, and both of these cases were late registrations.
I have read conflicting information about whether the parents would have had to pay a penalty for a late registration at that time?
For example, stories of parents adjusting a child's birthdate to fit within the 42 day window are ubiquitous, as described in this FamilySearch guide:
The father, mother, neighbor, or other person present at the birth must register a birth within 42 days. The 1874 act imposed a fee for late registration (43 days to 6 months). This penalty may have persuaded some parents to "adjust" their child's birth date to avoid paying the fee. After six months the birth could not be registered.
However, the text of the Registration of Births and Deaths Act 1874 (see section 39) seems to suggest the penalty was only for non-registrations, not for late registrations:
Every person required by the Births and Deaths Registration Acts, 1836 to 1874, to give information concerning any birth or death., or any living new-born child, or any dead body, who wilfully refuses to answer any question put to him by the registrar relating to the particulars required to be registered concerning such birth or death, or fails to comply with any requisition of the registrar made in- pursuance of those Acts, and every person who refuses or fails without reasonable excuse to give or send any certificate in accordance with the provisions of the said Acts, shall be liable to a penalty not exceeding forty shillings for each offence; and the parent of any child who fails to give information concerning the birth of such child, as required by the said Acts, shall be liable to a like penalty ; and a person required by the said Acts to give information concerning a death in the first instance, and not merely in default of some other person, shall, if such information as is required by the said Acts is not duly given, be liable to the same penalty.