The Rd and number within a rectangle on the outside bottom of such Swans would be imparted by the bottom plate of the mould during the pressing process (such in raised characters on the inside bottom of a pressed item would be imparted by the plunger). The Rd info. could be on the bottom plate from the first time the mould is used, but it's a simple matter to add it to the bottom plate later on if need be. And, it's possible that there could be two or more different bottom plates. Given the rectangle and the even line of numbers, I'd suspect this was chipped by a skilled mould maker, not done with a die stamp.