The Centralab R2-54 Fastatch Type B is a rear dual volume control unit designed for modular potentiometer assembly in vintage electronic equipment. It features a C4 taper specification with a control base diameter of 15/16 inch (7/8 inch). The overall shaft length measures 4 1/4 inches, incorporating a 1/4-inch flat inner shaft, a 3 1/4-inch top shaft section with a 3/4-inch round outer diameter, and a 1-inch bottom shaft section with a 1/4-inch round outer diameter.