Regarding Type J development with Ron Olson, Phil Crosby recalls:
I was helping him to pursue parasitic oscillations (we used to call it "bird hunting"). It was common to say when an amplifier had a parasitic oscillation to say that it "sung". At one point I was peering closely at a J unit prototype while Ron was probing it and a finger on his left hand touched the +225. The reflex caused the back of his hand to strike my nose pretty hard. Damn near bloodied it. Vacuum-tube voltages and solid-state devices had an uneasy relationship. In those days, a common probing tool was the point of a #2 pencil.
The Type J's tentative announcement was in 1963. It appears to be similar to the 1A1, which was introduced in 1964. The 547 training guide, dated December 30th, 1963 mentions "a new dual trace plug-in, the 1A1 (also known as the J)." Note: The Type J does not have a spoiler switch hole in the rear panel, while the 1A1 does have such a hole.