In another post, I wrote about a vintage, round badge Star snare in blue sparkle which I received as a gift. The drum is pristine! After disassembly for cleaning and polishing, I noticed an elongated diamond shape about 1 1/8 inches long stamped in the inner shell in blue ink. Within the diamond were the letters, "I.S" also in blue ink. The shell is a typical three ply with r-rings. The logo has me puzzled though. Any ideas? Thank you as always.

Brian