I think that some of the "single letter" marks on jugs and crocks (especially Albany slip items) were to identify the merchant where they originated from and were intended to be returned to. If you went to the store for a gallon of vinegar and didn't bring your own container, the storekeeper would loan you one.
Most small towns might only have one or two stores and a simple capital letter would suffice to prove ownership.
The hand turned Albany slip jugs generally date back to pre 1900. After that the zinc glaze finish started becoming popular.