Phonograph needle, Astatic N904-sd, replaces Tetrad A1 B1 C1 51 61 K1. Used in: Tetrad 51D 51S 61D 61S A1 B1 C1 Example: 2-51D-M10; 3-61D-34. Tip.7 mil diamond and. 3 mil synthetic sapphire. Speeds: 33, 45 & 78 RPM.