Covington: During WWI, there was an added tax on parcels which had to be paid with documentary revenue stamps (in addition to the normal postage). Some cities precanceled revenue stamps locally (the series beginning with R228), to facilitate payment, which is what I believe your reference is to. It would look similar to this R229 from Indiana:

Potatoes: The Federal potato stamps are from 1935, anything from 1918 sounds like a state revenue stamp of some sort. Illustrations with your questions would be so very helpful.