Pieces of that type are typically quite old, and from a time when there were hundreds of small foundries in operation. Markings in the form of a raised letter or letters were usually made at the time of casting by a molder, who impressed his initials, likely, into the sand in order to tally his day's production for payment by the piece. Knowing what foundry and precisely when is often uncertain, although sometimes design characteristics can narrow down a geographic region or a rough era.