Another expanation comes also from old British pubs. The phrase "mind your p’s and q’s suggested the barkeep keep an eye on his pence (pennies) and quids in the till.
Welcome to the SDMB, Randy.
A link to the column you’re commenting on is appreciated. Providing one can be as simple as pasting the URL into your post, making sure to leave a blank space on either side of it. Like so: http://www.straightdope.com/classics/a3_264.html
You may be interested in another recent thread on the same subject of Ps and Qs: Minding your Ps and Qs
What about the shillings? Thius one’s highly improbable.