Ah, ok, now I see...
No one ever responded to my earlier question
here about the precise function of this button (Store Master Password for All Accounts)... so, I'm guessing from this response that the answer would be the latter - stores the Master Password Hash for ALL ACCOUNTS (what the description on the button itself says, as opposed to the label above it)?
If so, then I don't see this as a good way to accomplish the goal... why store the same Hash umpteen times, once for each account? Why not just create a pointer to the Hash to the Hash for the Defaults Account? Then just make the 'Store MPW for all accounts' an option in 'Global Settings'.
Last, to store the Default Master Password Hash - just select the Default Account, then click the single button. Since it is context seneitive, it stores it for the Default Account, and this is the Hash that will be used for ALL accounts if the Global Setting 'Use Default MPW Hash for all accounts' is checked.