Centralab (CRL) R2-61 Fastatch Rear Dual Volume Control Type B, Tap 100K, outer shaft diameter (upper) measures 3/4 round (a third way from top to bottom), inner diameter 1/4 inch flat, top portion of shaft measures 3 1/4 inches and second section of outer shaft diameter (quarter way from bottom measures 1/4 inch round, bottom section of shaft measures 1 inch; control base diameter measures 15/16 inches (7/8") by overall shaft length measures 4 1/4 inches long.