He was referring to Q4,Q7,Q10,Q13 on the A17 board and I clearly remember them being thyristors
I dont remember their part numbers but 2N4101 rings a bell
one of them is different than the other 3 but they are all thyristors and are parallel to the supply rails for over voltage protection
there is another thyristor on that board which is a can shape (TO-205 perhaps) for the 50V rail
all pass transistors are mounted on the right side of the board on a big heatsink.
