It wasn't until 1979 that the Surveyor of the Queen's Pictures, Sir Anthony Blunt, was publicly outed as the fifth member of the Cambridge spy ring that the KGB dubbed its 'Magnificent Five'.
Blunt's unmasking shocked the public even though the British establishment had known for almost two decades that he and John Cairncross were the duo who along with Kim Philby, Guy Burgess and Donald Maclean had betrayed MI6's espionage secrets to the USSR.
The Smithwick Tribunal which is scheduled to resume public sessions in under a fortnight's time is beset with its own dilemma concerning who was the 'fourth' and perhaps the fifth IRA spy within the Garda.
Chief Superintendent Roy McComb detonated the equivalent of an artillery shell on the floor of the tribunal in July just as many lawyers and Irish government ministers were heralding its conclusion.
The consequence of the material McComb brought to Smithwick indicating that there was a hitherto unsuspected fourth garda who helped the IRA around the time Chief Superintendent Harry Breen and Superintendent Bob Buchanan were ambushed and murdered has rocked relatives and lawyers alike.
For Judge Peter Smithwick the implications of Chief Superintendent McComb's five pieces of intelligence delivered at the last stage of the Tribunal's public hearings are immense.
One analogy that John McBurney the lawyer for the Breen family uses is that of the final day of evidence during a murder trial when a prosecutor announces that he has more witnesses who may give evidence which does not entirely point to the accused being the perpetrator of the crime but insufficient time remains to hear their evidence.
"There is no conceivable way that this tribunal can be concluded now without more detail being given, if not in public but to the tribunal's lawyers at the very least, by the PSNI about the 'fourth' garda. Whatever the financial and time constraints that have been placed upon the tribunal by the minister Alan Shatter, what Roy McComb brought to the tribunal must be defined and the conclusions presented and tested in public", he said.
The implications of there being a fourth rogue garda hitherto unexposed or undetected by the Garda Commissioner is huge.
Earlier during the Smithwick proceedings there was reference to what may be another bent garda who within Tribunal circles is referred to as the 'Monaghan Garda'.
It's not known if the Monaghan reference indicates the officer's county origin, or his station or work area base, nor is it known if this officer is the officer specifically referred to by Roy McComb when he appeared at Smithwick in July.
If the Monaghan Garda is not the retired garda officer referred to by the chief superintendent then there were five garda at least based around the border who during the 1980s are alleged to have assisted the IRA.
Whether the tribunal resumes its task of interviewing witnesses like Chief Superintendent McComb remains unclear but it is imperative that the existence of the fourth suspected rogue garda and perhaps a fifth is explored.
That may necessitate the recalling of many Garda officers who served along the border who have already given evidence and who might be advised to arrange legal representation before they are brought before Judge Peter Smithwick again.
Those who have given evidence to Smithwick have immunity from prosecution in the Republic in relation to matters of a criminal nature they disclose in which they may have been directly involved. But there is no immunity from criminal proceedings if a witness is deemed to have committed perjury.
Where the PSNI obtained this information indicating a fourth rogue Garda officer so far unmasked is an intriguing analysis for another day. Chief Superintendent McComb's bombshell has created a major headache for the tribunal, the Garda Commissioner and the Irish government.