The History of the Legendary Marshall 100-watt "Plexi" …
MKII was used for quite some time on all Marshall "Plexi" panels after the JTM45 prototype and initial production run in late-1965. It wasn't model specific though — thus, as a result, you'll see it on old JTM45s, 1987, and 100-watt "Plexi" models. And on the current JTM45, 1987X, and 1959HW reissues, too.