Someone I communicate with on another discussion board has indicated the mark may be a Spanish "sigla." If so we may be able to identify the merchant who owned that sigla. If the mark is a sigla, that indicates to me the mortar was shipped as surplus, or scrap bronze. An artillery piece still in the official Spanish ordnance inventory would not, I think, have a sigla engraved on it, since the Spanish royal cypher would indicate ownership of the piece by the Spanish crown.