Every register therefore has its OWN index. Someone born on 15th June 1876 may be in the book that finished on the 30th June or may be in the next book which started the same day or the next. It would be quite possible for a registrar to complete more than one book in one quarter - my current rate is to complete one book of 300 birth entries every 7 weeks and the other registrar does the same so a reference that gives only the quarter might require 6 or more indexes to be looked at. The biggest office in England and Wales has 26 registrars and therefore a potential for about 90 indexes to be looked at for any one event. Nevertheless most register offices would look for and locate a birth or death entry if given only the GRO reference.
Marriages are a different problem.
There are a multitude of registers from a multitude of different churches and therefore a multitude of indexes to be looked at. There are not usually composite indexes for all the churches, and the bigger the population in the district the more books and indexes there will be. With the exception of Birmingham that had all its indexes put onto the computer by a youth training scheme, the indexes are not computerised because the cost in hardware, software and time would be far too great for the usage. Every registrar is issued with a book of regulations which dictates what they should do in every circumstance. The instructions regarding the issue of old certificates is quite clear.