I've just picked this query up whilst browsing the board. My best guess is that the " may stand in place for the shillings. As someone who can remember 'real' money, before this decimal rubbish came in, we were taught, if the amount included pounds, to always include pence,even if there were none - the amount would be written as say £1.1s.0d.(and that's a guinea - racehorses are still sold in guineas, not pounds)
A sum of just shillings and pence was written as 1/6 or 1/6d.
This is why I think the " stands for the missing shillings - but I was taught to write £1.0s.0d.,if there were no shillings. I've not seen the " used as a substitute. Could it be a mark used in legal documents only, rather than everyday writings?