The printer's p/q explanation may be quite compelling ~ my first go at typesetting included "garben", which is the lowercase b/d thing Jim alluded to, but if it's true that landlords kept your tally on a chalkboard, perhaps the p's and q's to be minded are your pints and quarts. This alternate p/q minding may be more germane to the lyric [?] Anyway,

keep on pickin'
dennis