Hi Jim
The original entry in the index was 5a 135
At some point after the initial registration, there was an amendment to the original entry and the entry should read 5a 6473/s
However, whenever you want a cert from the GRO and the index is incorrect in some way, you must always enter what is writtern on the image, not in the trancsription or what you think the entry should be. It will be down to the GRO to supply the cert against their index whether it is right or wrong.
If their system won't accept what is written in the index, then order the cert without using the GRO reference, their system defaults to 'No' 'the GRO ref is not known' and you can order the cert without the number by quoting his parents names.
If you want to purchase a copy of that birth cert for John and entry 135 has been superseded by 6473/s, the GRO will send you what is on 6473/s as entry 135 is in effect voided by the later entry unless there is now a block on supplying the cert.
As to the death of Sandra, I don't know how true this might be but her age at death could have been 23days or 23hours and when the GRO index was compiled the 'days' or 'hours' was omitted leaving her age as 23.
The GRO are very helpful and according to your profile you are UK based so give them a call on 0300 123 1837 and discuss the superseded birth entry with them and they will take your order over the phone with payment by debit/credit card.