I emailed Find MY Past and asked the following question....
"Could someone please explain why this entry for Rebecca Miller (later Berzon) has 2 dates of birth?
What is the significance of the date in the address column 23/5/45 and why do BQ/RM 80 mean on the right?
Carol
What amazes me is why you thought they would know the answers to your questions.
All FindMyPast have done is scan and index a large document, they like you have never been allowed to see the document before hand yet you expect them to know all about it.

They gave you a reasonable explanation about the "BQ/RM 80" code and explained they did not have access to the medical information.
There are many alterations and additions on the register some with knowledge of the family can be assumed to relate to a specific occurrence such as a date of subsequent marriage or a date of subsequent death, but I would suggest it is asking too much to expect a company to have knowledge of such specific details.
For instance on my mothers entry on the 1939 there is a subsequent date in the address field in blue ink (25/11/1946 SAA. M.)
I assume that refers to her change of name as she remarried on 21 October 1946 in Scotland and it would take a while for the information to trickle through the system.
The is also an entry in the address column next to her son in green ink (7_12_57 then the symbol for a half ½ followed by something I cannot make out and the initials EDB) this could possibly be when he changed doctors as he went up to Edinburgh University in 1957) there is also a change of name in green ink.
It could be that the first notification of change of name they had came with him going to university, but that I will never know.
I do know that some amendments on the register were in blue, some in green and some appear to be in pencil by do not know if this is significant at present.
Perhaps when others have downloaded images we might get a clue to the significance of colour or whether it just depended on what pen was to hand at the time, maybe time will tell.
Perhaps someone who worked on the register when it was used by the NHS may know and reveal the details.
Cheers
Guy
PS I have also seen snippets of alterations in Red ink but these were under a closed record so don't have further details.
Green ink seems to be used for changes of name on re-marriage I have seen