A bonus super-tax
The windfall that the chancellor wants to capture is bankers' bonuses, as I said in my last note.
So what he has in mind - and what may be announced in Wednesday's pre-Budget report - is not a classic windfall tax, which would be levied on banks' profits.
Instead it would be a super-tax on bonuses over a certain level paid to British-based investment bankers.
The advantage of taxing these bonuses are first that they are likely to be pretty popular with more-or-less everyone apart from the bankers, if opinion polls are to be believed.
But also, as I mentioned in my previous note, taxing bankers rather than banks would not weaken the banks themselves, at a time when they need to accumulate capital.
That said, taxing the bankers may not be cost-free for the UK.
The UK may have become excessively dependent on the City and financial services, but it would not be great for the economic prospects of the UK if wealth-creating bankers and financial institutions emigrated to rival financial centres - for fear that the UK is becoming irredeemably hostile to them.
Which is why George Osborne, the shadow chancellor, did not pre-empt the government by announcing that a Tory administration would impose such a super-tax - although he did consider announcing an intention to impose a one-off special tax on both big bonuses and dividends.
That said if the government announces such a super-tax, the Tories won't oppose it.
As one senior Conservative told me, such a super-tax might be bad for the UK, but it would not be great for the Tories to oppose a tax that may give a nice warm feeling even to many of their core supporters.