I did and it seems to refer to minor coins (1 and 5 cent coins, the ones that were not silver). I understood that the amount in pennies is limited to 25 cents, but the amount you can pay in dimes, quarters, half dollars, and dollar coins is unlimited. And yes a merchant can specify what he will accept for something, but debts are different. “All debts public and private” would seem to mean all debts public and private.
Hijack: I once dated a girl named Penny, so named because her parents had paid the obstetrician’s bill with a jar of 1500 pennies (this would have been the late '30s).