In November 1967 the Labour prime minister, Harold Wilson, took to the airwaves to announce his government's devaluation of sterling. From now on, he said, the pound abroad would be worth 14% less in terms of other currencies. "It does not mean," he added reassuringly, "that the pound here in Britain, in your pocket or purse or in your bank, has been devalued."
It was rightly seen as cynical sleight of hand, an attempt to put a positive spin on a very bad story. And more to the point, it lacked credibility. Even the man or woman in the street, untutored in the dark arts of the international money markets, understood that devaluation was not at all a good thing for the pound in their pockets.
But Wilson understood that the pound resonated with voters, as does Alex Salmond. Having long denigrated sterling (he once said it was "sinking like a stone") and even flirted with euro membership, Scotland's first minister eventually realised that ditching the pound was just too risky. The pound was one element of the ancien regime most Scots were reluctant to part with.
So Salmond made it his own, embracing monetary union while rejecting the political union that currently goes with it. A formal "currency union", he posited in plausible tones, would quickly be agreed post-independence because it was in the "best interests" of both countries.
Unionists, however, didn't play ball. Last year George Osborne hinted that the Westminster government would reject such a union, although such an indistinct stance left the ball in the SNP leader's court. Opinion polls suggested a majority of Scots and those in the rest of the UK saw it as the best option, so the Scottish government's line held.
But with the chancellor's speech in Edinburgh today all that has changed, changed utterly. The announcement, prematurely briefed to the BBC late on Tuesday night - that Westminster would veto a currency union in the event of Scots voting yes to independence - wasn't unexpected. I understand that such a move has been planned since the middle of last year, which explains the no campaign's sustained assault on the currency question.
It also throws down the gauntlet to nationalists to reveal their plan B for an independent Scottish currency. This they have been reluctant to do, instead arguing that the volume of Anglo-Scottish trade and closeness of the economies of Scotland and rUK (the rump United Kingdom left behind by independence) makes plan A the only credible option. Indeed, the SNP's opposition to monetary independence has always looked a little odd in the context of a debate about political independence.
That said, the UK government's strategy (soon to be backed up, importantly, by Ed Balls, the shadow chancellor) does carry risks, most likely short-term ones. The image of out-of-touch Westminster elites "bullying" Scotland will resonate to an extent north of the border - though, unionists calculate, only with those who long ago decided to vote yes.
The move hinges, rather like Wilson's "pound in your pocket", on credibility. If, as looks likely, the seven-month runup to the referendum consist of Salmond, Nicola Sturgeon et al saying "They're bluffing", and Osborne, Balls et al saying "No, we're not", then the former position is unlikely to pass the burden-of-proof test, which rests squarely with those in the yes camp.
Scotland's finance secretary, John Swinney, might claim that "people know" the "Westminster establishment will say one thing before the referendum" but behave "far more rationally after a yes vote", and perhaps that is true, but it's a risk a lot of undecided voters are probably unwilling to take. Pro-union strategists say they've tested this scenario "to destruction" via focus groups and are confident it will have the desired effect. As one insider put it to me: "People kept saying they wanted clarity. Well, now they've got it."
Talk of "threats" from the pro-independence camp is also ironic given that Salmond has been hinting for months that an independent Scotland might refuse to accept its share of the UK national debt should the Treasury refuse to play ball on a currency union. But again, this move fails the credibility test: a newly independent Scotland, whether nationalists like it or not, would have to operate within (and borrow money from) the international markets, which would not look kindly upon debt refuseniks.
As in so many other areas, the SNP only thought seriously about the ramifications of retaining sterling, having already announced it (longer ago than most observers think, in 2005). On closer examination, the policy required endless squaring of circles, a task delegated to the Scottish government's fiscal commission.
This held open the prospect of adopting a separate Scottish currency after a transitional period involving sterling, but that was not something the SNP was willing to countenance, despite other elements of the broader Yes Scotland movement being openly supportive - particularly those on the left. SNP strategists calculated, rightly, that Scottish public opinion would be rather less keen.
So the first minister, frequently written up as a master tactician, ended up boxing himself into an unsustainable position, and one that has culminated in the Osborne gambit in Edinburgh.
In many ways, Salmond has modelled himself on Harold Wilson: media savvy and tactically shrewd, but also opportunistic and ideologically indistinct. This only takes a freedom fighter so far; the "pound in your pocket" is one aspect of the independence debate that voters intuitively understand to be important, and it's territory on which the Scottish government has always been weak.
As Keynes put it: "He who controls the currency controls the country." It now falls to the SNP leader to prove that adage wrong.
David Torrance is associate director of Dundee University's Five Million Questions project, and author of The Battle for Britain: Scotland and the Independence Referendum