Some of the documents on Britain's role in this are missing from the volumes. In their place for many years was a single sheet saying 'Closed until 1989'. When 1989 came around, however, the papers were not released. Instead, the words 'Retained under section (3)4' were added, which means that the papers were inspected by government officials, and the decision was taken to withhold them from public inspection indefinitely.
What can these papers possibly contain to justify keeping them secret after 54 years which have seen the upheavals of a World War and a Cold War? What could British diplomats or spies have been up to that might still be sensitive today? Until Whitehall changes its mind, there is no way of knowing.
The case of the Sudeten German papers is one of hundreds which have turned up in the first attempt by British historians to catalogue the public records that they would like to read but are not allowed to, even though the usual 30-year period has passed.
The initiative, organised by the Institute of Contemporary British History (ICBH) in London, comes in response to an offer made last June by William Waldegrave, the minister responsible for open government.
Asked in a radio interview about the prospects for glasnost in public records, he said: 'I would like to invite serious historians to write to me . . . Those who want to write serious historical works will know, probably better than we do, of blocks of papers that could be of help to them that we could consider releasing.'
He has been taken at his word. With the support of dozens of Public Record Office regulars, the ICBH has identified a huge variety of closed files, and it will be reporting its findings to Mr Waldegrave at the end of the month. The meatier subjects covered include:
Nazi subversive operations in Britain during the war.
Pre-war Nazi sympathisers in high places in Britain.
Interrogation reports on German prisoners of war.
The abdication crisis of 1936.
The post-war Cabinet committee on subversive activities.
Sir Roger Casement, the Irishman executed for treason in 1916.
British Army action during the 1916 Easter Rising.
British irregular forces in Ireland 1919-21.
Sudeten Germans in Czechoslovakia in 1938.
Anglo-Iranian relations in 1953, the year of a coup in Tehran.
The attempt on Hitler's life in July 1944.
Full transcripts of intercepted German 'Enigma' signals.
Why is the Government keeping secrets about these things? The rules are clear: documents are released after 30 years unless they contain information which (a) might endanger national security, (b) might cause distress to individuals or their immediate descendants, or (c) was given on condition of confidentiality.
In some cases, it is possible to identify one of the three factors. The abdication papers, for example, are thought to be withheld because of possible distress to the Queen Mother. And immediate descendants of Irishmen who helped the British forces during the war of independence might well suffer distress if identified. But Sir Roger Casement? Nazi sympathisers? The Iranian coup?
Professor Kenneth Morgan, who has been studying the Anglo-Iranian papers, says it is 'ridiculous' that he is able to read material up to the year 1951, but not after. This means that the extent of British and American involvement in the coup of 1953 against the Prime Minister, Mossadeq, is at least partly obscured. 'It makes a total mockery of the work of the historian,' Professor Morgan says.
The official body which is supposed to safeguard the public interest in all this, the Lord Chancellor's Advisory Council on Public Records, chaired by the Master of the Rolls, includes reputable historians among its members. When government departments decide to withhold papers, they must justify the decision, file by file, to this panel.
The council, however, is a victim of a classic paradox of official secrecy: its members do not have security clearance, so they are not allowed to see the contents of the files being discussed. 'All we can do is ask awkward questions,' says Valerie Cromwell, a historian who has been a member. 'We are meant to see that things are run properly, but it's difficult in these circumstances to do a good job.'
Historians assume, in fact, that many papers are held back for quite innocent reasons, some because of past qualms, now irrelevant, some because the resources do not exist to process them, and some simply because they have been forgotten. The ICBH has identified a large number of files that may fall into this category. They include:
The International Whaling Agreement of 1939.
Aberystwyth hospitals and civil defence planning.
Black-market activity in the Second World War.
Private housing in Crawley in the 1950s.
The Poor Law in the 1930s.
Anglo-Swedish relations in 1941.
Food production in the Second World War.
Relations between Turkey and South Korea in 1957.
What could possibly be secret about these matters? There is a tendency to assume the worst, and nothing feeds a conspiracy theory so well as the concealment of evidence. Historians argue that it is often in the Government's interest to release a file, if only to defuse speculation.
That was the case with papers on police activities during a strike at Tonypandy in South Wales in 1911. Because the papers were held back for 75 years, it was thought that they must contain damning information about the role of the then Home Secretary, Winston Churchill, in encouraging police excesses. When they finally emerged, however, it was clear they had been kept back merely to conceal details of special police overtime and billeting arrangements. A similar anti-climax followed the recent release of papers on Rudolf Hess.
There is another paradox about public records. Most of the knowledge about files that have been withheld comes from the indexes of the Public Record Office, where tantalising blank spaces appear beside file headings.
But what about the cases where even the file headings are absent? And the papers that have never reached the index? These are the successful cover-ups. Lewis Johnman, another historian involved in the ICBH initiative, sums up the Catch-22: 'We don't know because we can't see, and we can't see because we don't know.'
Dr Johnman has been frustrated by one of the oddest secrecy cases of all. Four times in the first half of this century, the Government conducted a census of production, an all-embracing survey of British industry, for which every manufacturing company submitted details of its operations. Digests were made public, but the raw, company-by-company returns were not, because firms insisted that the information must not find its way to competitors.
Dr Johnman was interested in the raw returns, potentially a peerless record of industrial decline, and he thought that, at least in the case of the 1907 and 1912 censuses, the confidentiality factor would no longer apply. The Government explained that since the original promise of discretion had been made to the British industrial organisations of that time, it was the Confederation of British Industry that should have the last word. The CBI said no. Dr Johnman, and a number of other economic historians, are hoping that Mr Waldegrave will now change its mind.