There are so many modern replica bullet molds and powder flasks out there that if you are going to "mess with" antique Colt bullet molds or flasks you should get a copy of either edition of Robin Rapley's book about Colt accoutrements, In answer to your inquiry, I will quote from the first edition of Rapley's book, pg 219: "On both brass and iron molds a single letter will at times be stamped, i.e. A,D,H,S, and many more. These are believed to be "in house" markings, maybe quality control inspectors marks."