IT was a memorable piece of tabloid journalism. On September 16, two days before the referendum, the Daily Record splashed "The Vow". Presented on mocked up parchment, with echoes of the Declaration of Arbroath or Magna Carta, and signed by David Cameron, Ed Miliband and Nick Clegg, it was billed as "a historic joint statement".

It was nothing of the sort. The Vow said "extensive new powers" would be transferred to Holyrood under a timetable that had been set out by the the pro-UK parties the previous week.

It promised the Barnett Formula would be retained and, in response to highly contentious SNP claims that a No vote would damage the Scottish NHS, reminded people that health was already devolved. The rest was a cut-and-paste statement about the advantages of staying in the UK. Apart from the signatories, nothing was new.

All the pro-UK parties were promising more powers and had, by then, agreed to reconcile their differences and produce a joint package through what quickly became the Smith Commission.

In the end they agreed to hand Holyrood near full control of income tax and significant powers over welfare among other things - an "extensive" package by any measure.

That, at least, is how the pro-UK parties see The Vow: a way of hammering home the message that a No vote did not mean no change; a promise kept.

The SNP see it very differently.

This week Alex Salmond repeated his claim The Vow had a decisive impact on the outcome of the referendum, swaying 10 per cent of voters. And he went further. He insisted it amounted to a promise of devo max, a set-up that would hand Holyrood control over all policy areas apart from defence and foreign affairs.

In an interview fully endorsed by First Minister Nicola Sturgeon, he said the SNP would demand what was promised during in the referendum if they held the balance of power at Westminster after the general election: full fiscal autonomy.

The move has changed the terms of the general election debate in Scotland, turning the clock back to referendum campaign. Under devo max, Scotland would raise all its own taxation and, with oil at $50 dollars a barrel, falling North Sea revenues would blow a huge (£18.6billion from 2016-19 to be exact) hole in the public finances. Whether the Nationalists are wise to open this up as an election front, the next four months will tell.

In the meantime, it's worth unpicking Mr Salmond's argument. The idea The Vow was as decisive as he claims is highly questionable. If it changed the minds of 400,000 people, you'd have to accept that almost every reader of the Daily Record was a Yes supporter on September 15 but voted No three days later.

There is solid academic research to support the view The Vow - or previous promises of further devolution, for that matter - did not change the game. The Future of the UK and Scotland project at Edinburgh University, drawing on polling by YouGov, found that two weeks out from the referendum 39.9 per cent of voters believed more powers would be handed to Holyrood following a No vote. In the final week, the figure rose to 40.5 per cent, suggesting The Vow had relatively little impact.

It's also clear the Vow did not amount to a promise of devo max, a system the pro-UK parties view suspiciously as "independence by the back door".

In fairness, Mr Salmond is not considering The Vow in isolation. He bases his claims on earlier comments by Gordon Brown, who, on various occasions spoke of a "modern form of home rule" and taking the UK "as close to federalism as is possible". This will have been interpreted as devo max, Mr Salmond argues, and that's how The Vow was understood.

But is that what really happened? Did the Record's loyal army of readers pick up the September 16 edition, take in The Vow and think to themselves: "Hmm. In the light of Gordon Brown's comments, this means devo max. That suits me. I feel safer with full fiscal autonomy, what with all that scaremongering about the currency. So I've changed my mind. Independence is no longer for me, I'm voting No?"

Somehow I doubt it. But that's what the former First Minister is asking you to believe.