a milk score, according to the OED is
a tally or other account of the purchase and sale of milk.
and the AHD has this to say about dun:
To importune (a debtor) for payment: a dunning letter.
n.
1. One that duns.
2. An importunate demand for payment.
the best i could do to answer your question would be to combine the two definitions. if that’s the case, i’d say your explanation above is as good as mine!
david