'These are going to be instances where the original registration has been amended in some way, requiring an “occasional copy” to be sent to the Registrar General (ie to supersede the original Quarterly Certified Copy).
They won’t be late registrations, which would be sent in with the Quarterly Certified returns for the relevant quarter
If a change has resulted in a re-registration of a birth, the new entry effectively supersedes the earlier one. The earlier entry remains in the index, but would have to be identified in the usual way and a copy of that earlier entry specifically ordered'.