The significance of the files made public in the last 48 hours is that they have delivered confirmation of what was once dismissed as a 'collusion conspiracy theory'.
They represent a substantial addition to the debate on how the Troubles developed and why violence lasted so long.
For the first time they give a dramatic insight into the scale of collusion and, crucially, how much the British government knew about it.
The 'Subversion in the UDR' document was written in August 1973 by military intelligence and Ministry of Defence officials, with one civil servant expressing fears over the questions that "are bound to follow once it has reached No 10".
They were obviously concerned that the then prime minister would be shaken by their report that 5 to 15% of UDR troops were linked to loyalists and that the regiment was the "best single source" of weapons for loyalist paramilitaries.
So what questions did Downing Street ask?
We know that the shocking reports of subversion reached the prime minister's desk.
This is confirmed by a further document, stamped 'confidential', which records a meeting in September 1975 two years after the subversion document was written.
The document is a summary of a meeting where the prime minister at the time, Harold Wilson, and his secretary of state for Northern Ireland Merlyn Rees, brief the leader of the opposition, Margaret Thatcher, on political and security matters.
The five-page document shows that the politicians held a meeting in the House of Commons where Mrs Thatcher was brought up to date on events in Northern Ireland.
It is clear the discussion was frank and wide-ranging. They discuss the IRA, the performance of the courts and the overcrowding of prisons.
However, a key section on the security situation confirms that Downing Street was aware of the UDR subversion and reveals additional concerns over the RUC.
"The Secretary of State said that he was more worried by the current sectarian murders than by the bombings in Belfast," reads the document.
"Unfortunately there were certain elements in the police who were very close to the UVF and who were prepared to hand over information, for example, to Mr Paisley.
"The army's judgement was that the UDR were heavily infiltrated by extremist Protestants and that in a crisis situation they could not be relied on to be loyal."
The record of the discussion sheds no further light on these revelations. None of the politicians are recorded as raising concerns or expressing surprise at the comments.
Two years after the subversion document was written, therefore, the UDR remained "heavily infiltrated" and Downing Street was linking elements within the RUC to the loyalist UVF.
With this in mind an extraordinary RTE documentary filmed in 1977 gives a revealing insight into the UDR at the latter stages of the decade.
The programme interviews a number of senior officers, including an unnamed company major.
There is no suggestion that the officer was involved in any wrongdoing whatsoever but his comments echo those of his colleagues on the programme, offering a valuable insight into UDR thinking.
The TV reporter and soldier discuss the UDR's role.
Interviewer: "Who do you see as the enemy in your area?"
Officer: "Well the Provisional IRA is the only enemy we have."
Interviewer: "Are you suspicious of the Catholic community in your area?"
Officer: "Well a certain amount of them you have to be suspicious, because a lot of them are involved in the deaths of members of this company."
Interviewer: "Well could you give me an idea of how many Catholics in your area that you are suspicious of?"
Officer: "Well that's a very difficult question to say, well roughly speaking say 50%."
A nationalist resident tells the programme: "Obviously there are very good and very decent men in the UDR but the record has been poor.
"The great difficulty I see with the UDR is it in fact brings one section of the community into the security forces and keeps the other out."
Through the 1980s and 1990s, the Stalker and Sampson inquiries examined allegations of wrong-doing in the RUC, while three investigations by John, now Lord, Stevens probed allegations of security force collusion with loyalists.
The British government never allowed the inquiry teams to publish their findings.
Now these files unearthed from the government's own records have confirmed that as early as 1973 it was aware of large-scale collusion.
We know that politicians who asked questions were misled.
Crucially, there is no evidence of any substantial effort by government to combat collusion.
These files amount to a few dozen pages, but they have confirmed allegations that were evaded throughout the Troubles.
They leave us with one important unanswered question: What is in the files we have yet to see?