Yes, there is a need to have insulation between the screw, the "hood" and the buffer plank on the one side and the tongue soldered to the yellow cable on the other side. If not insulated, you’ll create a very direct contact between the yellow cable and the track bed of the M-track and in other words connecting the yellow cable directly to the brown return cable.

On my old layout I had a couple of those stop blocks (and of course at places difficult to reach) and I have dropped the small plastic insulation a number of times when changing bulbs, but luckily always found it after some searching after I had detected the short circuit. (I often did this type of work with the transformers switched off on that layout).

If the plastic piece is totally missing, perhaps you can wind a small amount of electric tape around the tongue soldered to the yellow cable at the end where the screw goes.

This solution worked for me too. Thank You very much

Jim