This is a sketch of the 728.5 to show how the pin hole is configured. You can see that you have to use a pin with a very unique shape for this to work. So to put the insulator on the pin you have to push it in half way, turn it 90 degrees, and then push it in the rest of the way. Therefore the bar could still be needed as in the 134 if in fact it was used to stop the insulator from spinning upon removal of the plunger.

Use the "View Original" link at the top of the page to see this in more detail.