No, not any kind over imagined political conspiracy, as usual, the truth was less exciting, more mundane, though in this case potentially serious.
Intelligence agencies had picked up indications of a plot by Palestinian terrorists to down airliners, (probably, for them preferably) El Al ones, using SAM-7 missiles, Rome and LHR were seen as the likely firing points.
SAM-7, like another Russian military product, was the AK-47 of the shoulder launched, heat seeking anti air missile world.
The US, also in the 1960's, built the similar Redeye (the IRA tried to get hold of them), the British a bit later would build the command guided, bulkier, faster, had to be used by well trained operator, Blowpipe.
But the production, distribution of SAM-7 dwarfed them all, this before the Chinese made unauthorised copies.
From wars in Africa to SE Asia, SAM-7 was the entry level surface to air system, plug in the nitrogen pack to cool the missiles infra red seeker, point at target, if the cooled seeker locks on to the engine heat of a target, denoted by a warning light, fire.
A slim missile pops out of the shoulder mounted launcher, the rocket ignites, at a high subsonic speed the missile homes in on the engine heat.
As long as the lock on occurred while the missile seeker was pointed at the target engines jet-pipe, as long as it did not decide to instead now home in on the Sun, or a reflection of the sun in water.
So you'd have to be bombed or strafed by an attacking aircraft then fire at the departing jet.
Not a problem when the target was an airliner.
SAM-7 soon became very vulnerable to decoys such as flares ejected from an aircraft, again, not a problem with an airliner.
So the prospect of terrorists having a SAM-7 in the vicinity of an airport would have made the blood of security leaders and politicians run cold.
Fresh in the memory then was the spate of airliner high-jackings, bombings, this seemed an inevitable next step.
How best to counter?
Clearly no chance in the case of LHR to secure all potential firing points along the flight-paths, the SAM-7 was limited to several thousand feet, but it was much more likely to both lock on and prove lethal for an airliner on take off or landing, preferably take off with the higher engine power making a better heat source.
So when you consider this, the deployment of the army in early 1974 makes perfect sense, they would patrol around the most likely firing points, (also, less visibly to the press, beyond the LHR perimeter), seeking if not to the catch an attempt in the act but also to deter an attempt.
The age of the suicide terrorist had not yet arrived.
Though armed police (and unarmed) would have supported this operation, in 1974 only some specialist marksmen and Colt Revolver carrying Flying Squad detective and Diplomatic/Political Protection offers were routinely armed, the specialised armed response teams of today's police were not around then.
A few years after this, a novel was published where Mid East terrorists use a SAM-7 to down an airliner from the roof of a London building, the DC-10 crashes on Victoria Rail Station.
The novel was called SAM-7 .
(Might be worth a look an Amazon et al).
A SAM-7 did down a Rhodesian Viscount airliner in the 1970's, more recently we saw that near catastrophe when a DHL A300 was struck in Iraq.
The IRA, courtesy of Libya, got a bunch of SAM-7's in the 1980's, since one shipment was intercepted this led to the wholesale fitment of decoy gear to N.I. based helicopters, military transports and VIP aircraft, C-130's, VC-10's, BAe-146 and BAe-125 aircraft. Though the IRA never successfully fired one at a target - military choppers in N.I. started operating in pairs and door mounted machine guns were mounted as a response to the news of the SAM-7's.
Throughout the intervening years, as stated, regular exercises were carried out to test the response of the police and army.
Only in 1974 and in 2003 was it a response to a perceived threat, the latter also had fears that a RPG-7 might be used to fire at a taxing airliner.
The reason you sometimes saw light tanks or armoured cars was that the nearest military barracks with vehicles having the comms and command and control facilities, was the cavalry unit at Windsor, rest assured the 30mm or 76mm weapons would not have been loaded!
As stated, the infantry came from Hounslow Barracks.
The idea was the 1974 deployment was part of an imagined coup attempt, is as nonsensical as the idea the the 2003 one was some kind of political move to drum up support for the upcoming Iraq war (surely it would have the opposite effect?)
But some just have to add 2 + 2 and make 5, or even 10.
[Edited 2010-07-24 10:57:18]