Genuine Marshall replacement knob which features a black body, gold cap, and a round push-on profile.  

Ideally these suit models 2203, 1959SLP, JTM45 2245, 4100, 1962, 1987X, 1974X, 1959HW, 2061X and more. However, it may fit other amplifiers with round pots. 

Marshall PN KNOB-90061 (Was Knob-00013)