A friend picked up a Baltimore Arms Co. 16 gauge double. It is in very good condition and appears little used. Barrels are 30" long and are damascus. The following markings are on the barrel flate. Can anyone provide info on what they mean? Thank you!

Do you know how to interpret the few barrel markings that are on this gun? Each barrel is stamped "SD&G", and "A". The receiver is stamped with the same "A", and "Pat 2-13-1900". See the attachment.