I looked a number of the microfilms of the EMI archives held at the British 
Library and noted the systematic way in which masters were destroyed - It is 
true that EMI kept a copy of every disc issued but these have been destroyed 
or jettisoned at various times over the last 100 years so again only a 
selection survives and is by no means anything like complete. 

The destruction of masters is rather complicated however a number of files 
do survive showing the process of selection and destruction and it should 
possible to reconstruct this process. 

A number of Hanover ledgers for the period 1900 to 1914 came back to Hayes 
with a great number of masters as part of the WW1 reparations in 1922. As 
Western European record production was centered in Hanover until the Hayes 
plant production in 1907-08 the ledgers record the number of stampers made 
and the dates the masters were destroyed - interestingly this started very 
early -  As an example most of the 12 inch German band recordings made in 
1902 lasted on average about 3 years before being destroyed, an odd 
exception sometimes lasted 20 years or more, but by 1910 a great number of 
master had already been destroyed - I believe that any issued record, that 
is anything that had been allocated an issue number and had not already been 
destroyed did come back to Hayes but much of this large body of material 
came to be destroyed in the 1930s. Endless acoustic Russian, Italian, 
English, French etc  master went into the melting pot in the early 1930s - 
Caruso et al were of course kept but the rest went. For popular and ethnic 
music the destruction was wholesale - When 78s gave way to LPs in the early 
1950s it took about 10 years before these started to be destroyed in earnest. In the early 1960 over a two year period all Victor master were 
destroyed ie Rachmaninoff, Kreisler etc, presumably this was part of the 
break between EMI and RCA as what could EMI do with these masters.  

What is apparent is a weeding out process over time. Quite miraculous 
anything survives but it is really only a fraction of what once was and 
makes the records all the more precious. These are rather random 
observations whilst researching in other areas but hope it helps